Tesco Biere Speciale 8x250ml 'stubby' Bottles for £2.75. Which is 5% lager for £1.38/litre. (Also 20x250ml for £7.08, £1.42/litre).
I don't know if this is a temporary offer (doesn't say it is), but it's much cheaper than any other 5% lager I can see. I bought instore, and online price is the same.
I prefer Tesco Premium Lager cans at £1.53/litre, but for lager this isn't bad (better than anything else on Tesco's lager shelf, I think) and cheap is cheap. Even the utterly tasteless Holsten Pils 5% is more expensive.