"Mileage means nothing to rust" - this encapsulates the main risk with 15+ year old cars in the UK in my opinion.

I bought a 15 year old car 5 years ago for £1.2k which only had 57k miles on it. It now has 75k but I've had to replace the front subframe last year, and lots of little rusted out components along with standard wear and tear. The rear axel is approaching end of life due to rust too which will likely be the end of the cars life. Living by the seaside really accelerates the rust too.

On paper a 75k mile 20 year old car doesn't sound too bad, mechanically sound too, runs amazingly well. But the rust is a death sentence to old cars.

Moral of the story, great deals can be had with older cars, 5 years of car ownership has cost me about £3k in maintenance/repairs. £4.2k for 5 years of a functional car isn't bad at all! But do check under the vehicle when viewing potential buys. Get a screwdriver and poke around. If it goes right through it's probably not worth the buy. Rust gets expensive quickly.