Samsung, Panasonic and LG are basically the best 4K TV brands I think. Sony occasionally have a decent model at a good price too, but those three are the top dogs.
I'm not too up to date on this years models as I worked in the industry a few years ago, so I'd do some research and look at what the big three's best mid range models are at the moment. Sorry I can't really offer pros/cons on specific models, but there's many tech sites which do.
As for whether 4K TV's are actually worth it to you - well it depends. The absolute basics of 4K is simply that you have more pixels on screen (hence more detail) and you have a wider colour range (much much wider range, giving more depth and accuracy to colours on screen). With that said, as HsC has posted, not that much is actually broadcast in 4K yet (and who knows what happens in the future), however there are a few things, particularly if you have Sky Q. A 4K TV will also upscale any other signal it receives, so in theory the picture is better no matter what you're watching or playing. You will absolutely notice a difference if something is encoded 4K or has a 4K signal. HD DVDs, games etc. will all look very good.
If you have a John Lewis near you I would 100% recommend going down to store and seeing if they have any returns or others reduced. As I'm sure you know JL have a very generous returns policy which ends up meaning a lot of the TVs they have out and reduced have absolutely nothing wrong with them. Do check over them properly and get them to plug it in and turn it on etc. but you could pick up a slightly older model at a fraction of the cost, and still have a sales warranty from JL.