That's in great nick. Looks almost entirely original. £250 in today's market isn't too bad. Offer them £200 and see if he/she bites and they might at worst meet you halfway.
If it's the right size, colour, model, year etc. then it's a bike that will last a long time and can be used for almost anything so £250 isn't crazy money. There will be plenty of people on here who would have picked something like this for around £100 a few years ago, but those days have not entirely gone, but the bikes are now few & far between.
What's missing from what I can see is:
GT toeclips & straps
OEM GT AMÉ Antomical grips - looks like they have one piece grips & barends?
The rest looks great. Original Ritchey Megabite tyres, uncut bars, original saddle. You might even have the original inner tubes in there!!! The owner might have the toeclips & straps & grips squirrelled away somewhere perhaps so ask them about those items.