What can you get for these money? I don't follow UK computer market (this is like $520 in US), and this is not enough for CS laptop to last you for years. You are looking for
- a lot of RAM (so you can load it with VMs)
- a lot of hard drive (to store all these VMs and their files, and probably to multiboot)
- decent LCD (14" at least, and definitely not 1366x768 crap).
So, check your stores, and let us know what you can get for these money.
Writing code needs a lot of screen estate - believe me. As is a mouse, and an external monitor and keyboard for your dorm.
"Everything on external hard drive" - what will happen when this hard drive dies? Think about backups as well - you don't want your only copy of the thesis to be left on the coffee table on the street.