Recommendations

I bought a 2007 Corolla back in 2015, it's an English import and I've loved it. It's had its issues over the few years, but you come to expect that as a car ages. Over the last year, something seems to go wrong every other month and it's spent over 10 days off the road (so far) in 2019 alone.
About two weeks ago the mechanic said to me that although there's no immediate need to change it, I will have to change it by the time the year is out or the repair costs will soar. Since I've been so happy with it (bar the inconveniences when it's been on its holidays in the garage) I've not looked at other cars since I've bought it. So I need recommendations!

Current car - 07 Corolla, manual, petrol, 1.3L engine.

Budget 12,000 but could be pushed up to 14,000 if the right car came along. I drive about 20,000 miles per year so I want something that would last.

I like the look of the 2014+ Mazda3 but the ones I've seen online seem to be diesel. Is there much of a difference between petrol and diesel?

Ideally, I'd like this new car for as long as possible so it would be nice if it was a decent size with boot space for adding to a brood etc!