Car appreciation... the aesthetics, the engineering, etc

Is there a sensible way to hire a car in the UK for a week?

Need to ferry 3 of us from Kent to the lake district for a week in March, train currently going to cost 270gbp

Have you seen the rental car prices in the UK? A week is going to cost about 270GBP even if you rent a tiny shitbox like a Clio.

https://www.rentalcars.com/?cor=gb&preflang=en&prefurrency=GBP will search them all, but just remember that many of the smaller companies (e.g. not Avis, Budget, Hertz, Enterprise, etc) will have depots in the middle of nowhere and also try and stiff you on damage. So budget in Car Hire Excess Insurance from someone like https://www.insurance4carhire.com/ to mitigate that, mention that you've got separate care hire excess insurance, and take lots of photos before/after, etc.

I picked a random week (11-18 March) and Maidstone, Kent and you can just scrape in under 270GBP although most have limits of 700 miles (some have 1400 miles). Most of the cheap deals are with greenmotion who are on the trickier side of them to deal with (I rented from them a couple of years back and survived). Many of the pickup locations for Maidstone, Kent are actually London City Airport, so note the exact locations too.