Since last year, Sony put the price of a 12-month PS Plus subscription up to £49.99. So getting 15 months for just £34.99 again is a fantastic offer not to be missed. You need to be an Amazon Prime member to enjoy this fantastic price (need a free 30-day trial?), and you'll see this deal knock itself down to £34.99 once it's in your basket.
For comparison, the best price we sometimes see for the shorter 12-month subscription is around £35. Don't worry if you've already got a PlayStation Plus subscription as you can stack this one on top by adding the code as soon as you get it ensuring you have 15 months, plus whatever you already have.