When I had my off-licence we were allowed to sell drink that was past its BBE date, as long as we advertised that it was out of date to the customer when they purchased it, and also by way of poster/stickers etc on or by the product. We also had to knock the price down. If it has a Use By date (usually on food items) then you cannot sell them at all once past this date.
Strictly, it is an offence to sell a product even 1 day past its Use By date.
If a product has a Best Before date then selling it past it is a gamble if the product has went "off" in any way., e.g. stale, as the shop has then committed an offence by selling food "not of the quality demanded by the customer".
If the beer is a few days out of date, it will be fine.
Legally it is NOT ok to sell anything past its date, however you should be able to get away with it if you advertise it to be out of date. I see this happen a lot in outdoor markets in London and hey-they get away with it.