This offer appears to be in store only, as it is not shown online.
Tesco are currently offering 25% off when you buy 6 single bottles of ale or lager of 1 litre or below. 1 litre bottles of San Miguel Premium Lager are £1.88, so if you buy 6, they work out at £1.41 each and this equates to 80p per pint.
This is the real Spanish lager, which is 4.5% ABV. This is totally different from the 5% ABV San Miguel that is sold in multipacks of smaller bottles, as this is brewed by Scottish and Newcastle in the UK.
For comparison, the best current offer for bottles of Stella, is 40 x 284ml for £16 at Tesco / Asda / Sainsbury's and this works out at exactly the same price per litre. Granted the Stella is a bit stronger (5.2% ABV) and the big 1 litre bottles of San Miguel won't be to everyone's taste, but if you're looking for the real deal, this is a good price.