Need advice on which new car to get

My partner and I are looking to get a new car but we really don't know where to begin.

I think realistically we will be looking at used cars because my Partner does an 80-mile round trip each day Mon-Fri because of work. 2-3 years down the line, the car will be worth next to nothing so we really can't look at new cars.

Here are some pointers:
- a car that can endure constant long-distance driving
- preferably diesel (most the journey is motorway driving)
- preferably 1.4-1.8 engine
- something fuel efficient
- 5 doors
- preferably hatchback but can go larger
- we don't want anything too expensive, we're looking to spend around £7000 for a used version of this car so don't mind old models are suggested

Any help greatly appreciated!! We are car novices. :)
(our current car is a Vauxhall Astra)