Technically ADS-B receivers in the UK are probably illegal to use for the same reasons that you aren't supposed to listen to ATC air ground air traffic. The legal position can be found in a sticky at the top of the ATC forum.
In practice the authorities tend to turn a blind eye to listening to air ground air for private use and might do the same with ADS-B receivers. They usually take a less sympathetic approach to people who attempt to publish air ground air conversations on the web and those who stream ADS-B. For that reason you won't find any private sites covering the UK or if you do they tend not to stay up for very long.
For the obvious security reasons you won't find live UK radar data on any official site but many airports have WebTrak pages which show historical data, normally a couple of days old.