Mazda Premacy ?

You could try out the previous generation Honda Civic (Five Door).

I have one, and my not so agile parents can get in ok..

It's very spacious in the back, bit like black cab. This makes it very easy to secure Children into child seats, far easier than my wife's Ford Focus. It's quite easy for Children to move around in the car, including from front to back, so they can exit out of the safest door.

One adult and three children no problem, but you might struggle with three childseats in the back.

I have a 1.6SE petrol which I think is insurance group 6 (in old categories). So long as the previous owner looked after the car reliability is fine.

It's best to get a car with air con - they steam up very easily. Some of the earlier ones were made in Japan which means that one or two parts are a bit awkward to get (e.g. rear wiper blade). I should imagine a UK one would have an easier parts supply.

Dare I suggest a Doblo? You can an awful lot of Fiat for 4k. Nice high upright seats and tons of room.