There have been a lot of deals and discussion lately over Chinese and Other mobile phones. While researching the topic myself I found that a lot of the usual phone specification sites (Eg. gsmarena) lack details on the Chinese phones even when they are officially sold in Europe.
I'd looked for better sites and not found many. Today I delved into Kimovil, I think I'd googled to there a couple of times recently. Anyway it has a lot filtering options and seems to have pretty accurate specifications. Good filters for price and network frequencies.
This makes it much much easier to identify the foreign models that should work 100% with all the UK networks. Between them they use 4G on 800, 1800 & 2600 MHz. Here's a search for that to get you started: http://www.kimovil.com/en/compare-smartphones/fe_bands-lte-800,fe_bands-lte-1800,fe_bands-lte-2600