That's either the Early 2011 or the Late 2011 model. If the Radeon graphics card has 512 MB RAM, then it's the Late 2011 model. If it has 1 GB RAM, then it's the Early 2011 model.
In any case, if I feed the specs into Mac2sell.net then they estimate a £660 value. In this case, it's boxed and you get warranty as well.
I think it's pretty decent. It's not a bargain, it's a decent price. Note however, that if you want a nice performant machine, you need to install an SSD. That's going to cost about another 60 quid if you can install it yourself.