Samsung Galaxy s2 - Free for £25.54 per month(18 months) - 300 mins / 300 texts / 500 MB data on T-mobile @Affordable mobiles
This is probably the best 18-month contract. This will cost you £459.72 before £60 cashback(Quidco & TCB).
After cashback £399.72 which is almost equal to the sim-free price.
I am not sure how good T-mobile is, so I am waiting for your valuable input before placing my order.
This deal is not directly accessible from their home page. So please copy & paste the link after opening the website through quidco or TCB.
PS: I got the link from uswitch.com and modified it to avoid cashback problems.