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.
I agree with some of this, but do note that the base 2.3 GHz Mid-2012 non-retina came with 512 MB of VRAM, not 1 GB as all the Mid-2012 15" retina models did. The 2.6 GHz models came with 1 GB of VRAM. Also, note that the quad-core 2.3 GHz was an i7, not an i5.
The base 15" cMBP, if upgraded with RAM and SSD, will feel just as fast as a new retina model, and will be slightly slower on an absolute basis. Will you notice the difference speed-wise? Most likely not - if you do the sort of work where you would notice the difference, you probably need a different machine altogther, like a Mac Pro or a higher-end iMac.