It's my 30th deal so please be violent :)
Simple really - Cheapest I can see is £31 on electronic first (have used them many times before, won a free code from them on facebook!). Not only are you're 12 months renewed, but when you enter the code, 600 free points are entered into your account!
OR, you can get £29.99 for 12 months through the standard credit card service. If 2100 points is £15, then 600 is around £4, so I would go for the free points deal, but it's up to you!
To get this deal you need to be up for renewal fairly soon (within 2 months I think).