Last few times I've been in Tesco's they've been doing a £8 for 8 x 500ml cans offer on Old Golden Hen (4.1%) which is a very nice golden ale made with Galaxy hops. It's hard to find decent beer for £1 per can or bottle these days, I usually seem to end up having to pay around £1.50 for something decent so I've stocked up with this a couple of times now.
Another long-standing bargain is Lidl's Grafenwalder Pils (4.8%), very cheap (about 90p a can IIIRC, I last got it a couple of months ago so I don't recall exact price), it's a very refreshing lager as long as you like your beer very bitter! (It's not for everyone that's for sure!)