What most around here start with is a self assembly National kit. Thornes and Maisemore are the largest suppliers in the UK, look for what they call "budget" or "rock bottom". What you get, in cedar, is a deep brood, two shallow supers, roof, floor, crown board and queen excluder and all the frames and wax foundation. Price around 150 pounds delivered. Both offer 14x12 options if that's what you want for a bit more. The assembled option if you're not confident with the woodwork is another 40 or so. They are both established UK makers, reasonable value for a well known product, have a reputation for sorting out problems if there are any. Look for the delivery charges and what qualifies for free delivery to compare actual prices. Some prefer one or the the other supplier, I'd be happy with either. I'm not saying they are the only suppliers to consider, but it's a benchmark to compare others with.
Frames are usually self assembled from parts, it adds quite a bit over time to buy them all pre-assembled. One advantage of the kits above is that you get one hive worth of frames and foundation in the pack. Otherwise there are 11 frames in the usual National brood box arrangement but frames and foundation is usually sold in packs of 10. Not a problem to have some spare but first year it's easier not to have to sort out your spares storage too.
Caddon are an alternative, I know some who are happy with what they supplied. There are other local small scale suppliers around the country. The ebay guy selling assembled cedar hives for 85/90, with frames for 125 looks good value; not seen the product, just going on photos, description and brief feedback. If you're determined to get the cheapest option there are ply kits from Fragile Planet for around 50 quid, add your own glue and paint. That's plus delivery but two hives would be in the free delivery range. Probably not the cheapest for frames or foundation.
You pays your money, takes your choice. If you haven't seen hives in operation yet I'd strongly suggest you try contacting your local association. If nothing else, there are people there you can talk to about bees, hives and suppliers and get an idea about how things actually work out in practice.