Builders tend to use the cheapest cable they can get away with. It's very likely to be a basic RG6-type coax with a copper coated steel core and aluminium braid. Since it's indoors and partially enclosed within the walls then it won't get any weathering. It will be a bit more lossy than decent cable but there's not much you can do about that since it's enclosed within the fabric of the building.
When you come to make the hook-up lead for the loft and thinking ahead to moving the aerial outside then you can use some decent all-copper cable such as Webro WF100, Triax TX100, Labgear PF100.
One area I'd definitely pay attention to is any wall sockets. This, for two reasons.
First, with only a few exceptions sparks are generally pretty hopeless when it comes to aerials. Its worth checking they made the basic connections correctly. I have seen braid shield left unconnected, or braid wrapped around the centre core before fixing. They also have a habit of daisy-chaining RF sockets.
Second, they use unshielded sockets because they're cheap. Being unshielded is a point of weakness for digital TV signals. You can tell an unshielded socket because you'll be looking directly at a small circuit board. Shielded sockets have the cables connection point enclosed in a small metal housing.
The requirement for 4x TV points means I think you're right about needing an amplified splitter. I would consider using a masthead splitter with variable gain. Masthead amps don't have to be installed outside on the actual aerial mast. It is the best place for any amplified splitter since it's where the cleanest signal can be found with the amp closest to the aerial, but it can be left on the loft too.
This PROception 4-way has 1-16dB variable gain. The power for it is a separate transformer that uses one of the coax downleads as a power cable. That's handy if there's no convenient 220V power supply in the loft. Low voltage power and TV signal can co-exist quite happily within a coax cable. Satellite receiver systems use the same principle to get power to the LNB on the end of a sat dish. ProCeption PROMHD14V
One point to note, at least one of the aerial socket wall plates need to be able to pass the power in order for this remote powering option to work. In the trade we make the distinction between isolated and non-isolated wall sockets. You might also see them referred to as DC-Pass which is easier to understand if you know that the power is indeed a DC voltage. Isolated means that it blocks DC power. Anyone who may have used Sky IR repeaters (Sky Eyes) will be familiar with this.
Here's a link to a PROception shielded non-isolated RF wall socket. Their part number is PROSLO11C
Toolstation does PROception gear, but they don't list the manufacturer codes. You'd need to ask in branch or through their online help if this shielded socket at £2.50 matches the PROception part number or get something from ATV.
Regarding ATV and why they list the Log36 and Yagi in their loft kits, my guess is because they're both relatively compact. Folk seem to have a bit of a blind side when it comes to numbers and working out whether an aerial will fit a space.
