If you wanted upgradability, I would recommend finding a good-as-new 2012 15" non retina MacBook Pro. This comes this an Nvidia GeForce GT 650m (1GB GDDR5 VRAM and not that far off the new retina models GPU, but it has a quarter of pixels to push), a 500GB HDD, 4GB of RAM and a quad core 3rd gen i5 that is only a little slower than the current offerings. I've seen good second hand models of these go for around £400-450.
For upgrading though, the RAM could be upgraded 16GB yourself, and the HDD could be replaced with an SSD. You could also replace the he SuperDrive with an SSD, set both SSD's up in RAID 0/1 and have a,axing performance for around the same price as the cMBP but with 4X the RAM, double the CPU performance, dual SSD's, a larger 15" screen and a great GPU (million times better than HD 4000).
If you like upgrading, this would be heaven and save you some cash too