Sony in the coming weeks is going to announce their Black Friday line up. Some are good stuff and some are disappointing.
Anyway, PS Plus is going to be £37.49 from PSN Store from the 20th of November (date to be confirmed)
, so using PSN credit you can get it cheaper. In order to do that you'd need a £35 top up and to pay £2.49 on card.
Currently Shopto is the cheapest place to buy PSN Credit from but as things changes all the time I have put links to most common places selling top ups.