Well it probably will not be London for cheapness. I was paying around £4 a pint when there last month. Having said that London staple session beers like Youngs ordinary or London Pride were - as ever - excellent in the pubs I visited. Both are available all across London, but there are some very excellent pubs along and around Borough High Street, serving them. In the Market Porter, in Borough Market, and the Royal Oak, in Tabard Street (near to Borough Tube) you can also get the excellent Harvey's Sussex Best Bitter. In the Royal Oak there is the full range of Harvey's Ales and Beers. I also went to the very excellent TheCharlesDickens in Southwark, where there are at least six great beers on, and some good ciders too. I had a very very good pint of London porter, I think it was called Bayswater London Porter, but again they were all around four pound.
Outside of London, in deepest Sussex and Surrey there is still plenty of Youngs or Fullers to be had, plus of course the fantastic old King and barnes pubs, owned and serving Hall and Woodehouse's excellent beers. You might get lucky in some of these locations and get a pint for just over three quid.
The place I always remember being the cheapest for a pint of excellent beer was in and around Tadcaster/York for a pint of the very lovely Sam Smiths, but I am not sure what they charge these days. Not so very long ago you could get a pint for less than £2. By...but it's grim the t'north.
Finally, if price is the thing then you probably can't beat the excellent bottled beers sold in Lidls, and for price and extensive range Asda takes some beating too. Mmmmmmm I have only been back just over a week, and you've got me missing a good pint already...and Efes is not in any way a substitute for any of the beers mentioned above.