If you want a just an expensive phone, you can get them over there < 100 USD. but if you want to play games on it, its a different story.
look at Carphonewarehouse.com. The stores are all over London. Ive seen a share of them in Leeds too. Those are the only places ive been to so far. there are other outlets, but you can get an idea on price and whats offered. Remember $$$ is in pounds.
The PAYG sims are a really good deal. but if you want to add minutes, make sure you do it there or get some vouchers. Once you leave the UK, you wont be able to add minutes unless you have a UK credit card/address. At least that has been my experience.
If you want a tablet, i would look at COSTCO or some other e stores. at one time COSTCO had some wi-fi tablets for < 200 USD. Also, i think the carphonewarehouse had some tablets too so you can price compare. I will let you do that. The thing is that if you by something from overthere, the charges wont fit our outlets without an adapter.
If you get a wi-fi only table/device you wont have to worry about a sim card. However, im betting that wi-fi isnt everywhere as in the USA, so you will take a chance of not having access for a time. If you decide to get a device with cellular, it will cost more.
you can look on Craigslist, Ebay or look for a local store that sells used cellphones/tablets that are unlocked and GSM/quadband capable too.
just a comment. depending on the size of tablet you may get, it could end up being a PIA to lug around. I would spend some time window shoping in your area for tablets. Go out and pick them up and see if you want to carry the extra weight with you. At the least you can figure out what size of tablet you will want.