Its more expensive with the cost of labour in the UK to keep a car going, especially once it starts needing welding. We apply a lot of salt to the roads in the winter so after 20 years most cars will need welding work.
Second hand cars are also cheaper in the UK compared to other countries because we're right hand drive so there aren't as many markets that they can be exported to second hand.
I live in the UK. Cars aren't that cheap (at least for something half decent), labour costs are dependant on what is being done. While the roads do get salted, rust is more dependant on the vehicle model. Also there are preventative measures you can take that aren't that expensive again rust.
Second hand cars are also cheaper in the UK compared to other countries because we're right hand drive so there aren't as many markets that they can be exported to second hand.