In very broad terms, if you have/had a regular Sky/Sky+/Sky+HD set-up, then you'll be able to swap the Sky box for a Freesat receiver/recorder as simple as just swapping the cables over. Humax made decent Freesat PVRs, but I don't think they're still available to buy new any more. There are some changes afoot with Freesat.
Freesat is a joint venture between the BBC and ITV. They gave the contract to Humax to make Freesat boxes, but that agreement ended as of September/October last year. The new contract went to a French firm called Arris. They promised to make the next generation of Freesat recorder; the big feature being 4K compatibility; except they didn't. Not for a long time. It has taken over six months for Arris to get its act together and deliver Freesat recorders.
Their product line-up now is one receiver and three recorders ranging in HD size from 500GB, 1 TB and 2 TB. You'll find them online and at Currys as the Freesat UHD-4X range. All the recorders have two tuners available when connected to a dish used for Sky+/Sky+HD. This mimics what a Sky+/Sky+HD box could do which is either record one channel while watching another, or record two channels at the same time so long as you either watch one of them or play back a previous recording. All of the boxes will receive HD channels.
Change the LNB to a wideband version as used by Sky Q then these recorders are able to make four recordings simultaneously.
The 4K compatibility is limited to streaming sources rather than satellite channel reception. The boxes have a smattering of streaming apps such as BBC iPlayer, ITV Hub, Youtube and Netflix. If you subscribe to Netflix and can access their 4K UHD content then the Arris boxes can display it on a suitable 4K UHD TV.
Can I use my old Sky box?
Yes, but with some provisos. Firstly, Sky uses its own EPG channel guide, so it doesn't follow what you have on Freesat. Put more simply, you'll still have all the Sky channels to flick past in between those channels that you get for free. Next, you won't get as many HD channels. Sky repackages the HD channels as a cost option bundle. Once you're out of contract and no longer paying a monthly subscription for HD then these channels will disappear for you. It doesn't matter that they appear free on Freesat. Sky isn't giving you Freesat. It's giving you Free-to-Air satellite. That's a different thing.
The question everyone asks is "can I still record?" The answer to that is very likely not. There may have been some older boxes that missed out on a program update which means they could (for a while at least) still use the recording feature. AFAIK, that loophole has been closed.
Freesat pros and cons:
pro; the picture quality is a tad better than Freeview because it's not as heavily modified
pro; there are more HD channels on Freesat than there are on Freeview
pro; aside from the cost of the box, it's an easy swap to go from Sky to Freesat
con; you need a Freesat box for each TV if you're not going to get the aerial sorted
con; getting wiring from the dish to other rooms can be a pain
con; the standard 4-output Sky LNB only supports two recorders... and before you ask, satellite signal cables can't be split as you would with aerial signal cables
There's also a side issue that Freesat lacks a few stations found on Freeview. The most notable are Ch4 HD, All4, and the online streaming portal 4OD.
Freeview isn't perfect, but it might be worth you investigating getting a new aerial.
If you go for Freesat
