Where a TV picture pixelates it could indicate too much or too little signal. That might sound obvious, but the causes aren't the same.
In your case, the trigger is you, but you're not the root cause. You're simply changing the local conditions as you move about. The reason why your movements are having this effect is water. You're made up of about 60% of it, and water is good at blocking TV signals. It's the reason why trees are a problem for TV signals. It's not so much the break in the line of sight between the transmitter and the aerial - if that were the case then loft aerials wouldn't work at all - but the amount of water in the leaves and branches, particularly during the growing season; there's even a measurable signal level change with a loft aerial between dry- and wet- roof conditions.
For your install there are three main things going on. First, the aerial is in the loft rather than on the roof, so the signal level and it's quality will be a bit compromised compared to an outside aerial. The effect of this could be magnified if the aerial itself isn't a good match to the transmission frequencies of the local transmitter.
Second is how the aerial and sockets and building cabling have been installed. From your description, it does sound bodged. However, before condemning the installation site unseen, I have to say that there is a way to have a daisy-chain arrangement that does work. It's a system used in large buildings, and it uses high-power head amplifiers to condition and boost the signal, then small devices called Taps which look a bit like basic splitters to take a small portion of the signal for a local TV then pass on the rest to the next Tap point. This sort of system makes sense in a large block of flats or big hotel, but is OTT for a small multi-dweller property.
You might be able to tell a Tap system from a bodge by looking at the back of the wall plate. Where you can see two cables jammed in then it's probably a bodge. A Tap system would have a single cable leading from the Tap device. The Tap itself would be marked up with the signal flow (In, Out, Tap) and have a dB rating for the amount of reduction that the Tap provides, e.g. 8dB or 16dB or 24dB.
A bodged daisy-chain install is the mark of an electrician in my experience. They mean well, but often lack any understanding of the differences between electrical wiring for mains sockets and light fittings and what's required for a successful aerial installation. That's why the cabling is installed incorrectly, and too often they use the lowest grade cable and ancillaries too which then causes problems later on. Two things spring to mind immediately; the first is poor quality coax, and the second is unshielded wall plates. Both of these are is more vulnerable to picking up sources of signal interference. The knock on effect is that the margin between good signal and background noise is vastly reduced. In the trade we refer to this broadly as Signal to Noise Ratio (SNR). The worse the SNR then the less robust the aerial signal will be to variations in interference level, which is sort of what you're seeing as you move around your living room.
Finally, there's the quality of the extension cable tacked to the skirting. Winston1 has mentioned the quality of the plug connections. I'm going to focus on the cable itself.
I get called out to troubleshoot people's aerial systems. A fairly common complaint is loss of Freeview. Where there isn't a tuning or signal level issue from the aerial, then 9 times out of 10 it's due to a poor quality coax fly lead, and often it was something bought from a supermarket or DIY shed or lowest cost online retailer. These cables are invariably poorly shielded and very lossy. This means they allow in interference from things such as the power supplies in most home electronics devices and HDMI cables, and they lose a lot of the signal level which then makes the SNR much poorer. Replacing them with a piece of properly shielded coax cable is often the only action required to cure the reception issues. People buy extension cable kits made from the same crappy coax cable. The same can be true of unshielded wall plates.
Any length of poorly shielded cable can act as an aerial. That's what it's doing when it picks up interference. It can do the same for TV transmissions too. Your length of coax tacked to the skirting may be acting as an aerial, or the wall plate picking up signal, and your movement is changing the signal level going in to the TV.
So what can you do?
First, you need to isolate whether it's a signal level/signal quality issue. You don't have access to a professional aerial meter*, but most TVs have a basic display for Signal Quality and Signal Strength as part of the tuning menu. How to access this varies from brand to brand, so refer to your user manual. Downloads are usually available from the manufacturer's website.
Put the TV on one of the channels most affected by the pixelation, and then access the TVs signal metering. The thing you are most interested in looking at is Signal Quality. Amplifiers/boosters can increase Signal Strength, but there's no piece of domestic electronics that can be plugged in to restore Signal Quality. Once it's gone, it's gone. All you can do is to remove the device or reverse the setting that might be reducing the Quality; so for example, removing an amplifier that wasn't needed, or improving the quality of a piece of cable.
Some TVs give Quality and Strength as percentage readings, while others simply describe the condition with words such as poor, adequate, good, excellent. Nearly all TVs overate the aspect they're measuring so you'll have to read between the lines so to speak.
Adjust the position of the TV so you can see the readings for Quality and Strength. In an ideal world you're looking for Good~Excellent on Quality (80+%), and Strength needs to be somewhere in the 50~70% range, or Adequate ~ Good. Move around so you can see the signal levels changes affected by your position. What happens to Q and S as you move around?
Next, take the TV over to the wall socket and plug in the aerial cable directly. Repeat the tests. What happened this time? If you found that the problem didn't occur on the second test, then the issue is the quality of the coax extension. It needs to be replaced with better quality shielded cable.
Where the problem is the same in both tests, then I would look first to the quality of the wall plate. However, because other flats rely on the signal, and because it's part of the fabric of the building, then it's the landlord's responsibility to get this sorted rather than yours.
* the ones sold at retail for £10-£60 aren't up to the job. Don't waste your money.
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). Thanks for the follow-up, it's v kind of you. I'll post when I get home and see if the attenuator solves the signal quality/strength imbalance.