As things are starting to go wrong with your car, you’re unlikely to get your £450 back, so I’d say stick with it until something serious goes wrong and then scrap it. I’m almost sure your car doesn’t have a DPF, and that would be the only reason why you shouldn’t have a diesel for low miles, so just stick with it. When the time comes, I’d get something like an 06 on Toyota Yaris with the 1.4 chaincam diesel, which also has no DPF. Much more modern than your focus and also utterly reliable, as long as you don’t go anywhere near the automatic. A lot of people don’t like diesel these days, but as long as you tread carefully, you’ll get a quicker car that lasts for longer and costs less to run.

The brake calipers sticking is not always a symptom of an older car - many doing low mileage stick, some more than others. If its been reliable otherwise and you don't need to drive to a town/city that may soon be introducing a ban or heavy charges on more polluting cars (e.g. below Euro 4 petrols or Euro 6 diesels [yours will be below that]), then I'd run it until something expensive goes wrong or serious corrosion takes hold.

General consumables (including tyres) and the odd repair (like brakes) are far cheaper than having to spend many thousands buying a replacement car, especially a second hand car that may not have been driven sympatheitcally or as well maintained as the service history says. Besides, £800 isn't going to get you much, especially if you don't buy privately.

Your car is a known quantity.