I still use a prehistoric IBM ThinkPad, 1.1GHz, that I've had for something like 5 years. It's an excellent machine - certainly not state-of-the-art, but it's been very reliable. I replaced the HD soon after I bought it (wanted more disk space), added some memory, and added the heat sink compound that got left out when they built it.
I'd like a later model ThinkPad really, but can't come up with an excuse to upgrade - this one does everything I need.
£200 would get you one now.
The catch with all these is battery life - especially with older batteries. When it was new, mine would do close to three hours on a battery (depending on what I was doing with it). Nowadays, the battery is lucky to make 45 minutes. I've got mains and car (ie aeroplane) adapters for it, so I haven't bothered to try to trace a NEW replacement battery. (Never buy second-hand or old-stock new batteries!!)