Safeway:
Last time I was back, they had Franziskaner (white beer, and dark white beer), Erdinger and Schneider. The fact that a bunch of uneducated fools with business practices stuck in the 1930's are now in control of Safeway means that they now probably only stock Tetleys bitter, but its worth a look.
Waitrose:
If you can find a good Waitrose, they have their own branded whitebeer, in normal and dark varieties. It's actually a beer from Plattling, and is the dogs nadgers, but pricey.
Apart from that, its hit & miss. Some offies will have it , but even if one Unwins does, the next one down the road won't.
Best info I can offer.
Don Riina (who's mother fills the fridge from Waitrose every time he visits)