The description of this product is deliberately misleading and for that reason I have knocked 2 stars off. You are simply buying a credit / voucher which can be applied to your PlayStation account wallet balance. To get the PlayStation Plus membership, you need to buy it from the PlayStation website (using your credit balance or credit card).
The only reason I bought this was because the PlayStation website wasn't working for me. I only wanted one month at £6.99 so now I have £3.01 in unnecessary credit because I bought this £10 voucher. So if you want a PlayStation Plus membership, best to just try and buy it directly from the PlayStation website first and if it doesn't work for whatever reason, buy this voucher instead.