There's an environmental argument that it makes more sense to keep an older, diesel car on the road than replace it with something less polluting. If you do want to replace it, I wouldn't limit your search to specific models. See what's being sold privately locally, with a long MoT and evidence of regular servicing. A Ford Fiesta or Toyota Yaris could be a sensible option, or a Honda Civic if you'd prefer something a bit bigger.