I absolutely do not think I would make this swap.
You're going from a car which you like, which is cheap as chips to run with no faults, to an absolutely bottom of the line 10 year old Audi/BMW which could give you all sorts of headaches. No disrespect, but you don't sound like a "car guy" so it means that any faults these new cars may have (and they likely will have some) means you'll have to take it to a mechanic to fix, even minor things.
You only do 2-3k miles a year, so miles per gallon is pretty much irrelevant. My calculations are that your Focus costs you a mind blowing £374 a year in fuel. A 318i will cost your £306 - that's a saving of a whopping £5.67 a month. With the car tax factored in, this rises to a dizzying £14.64 saving per month.
Of course, a service on the Audi/BMW is likely to be more, the tyres will be more, the brakes will be more - and that's assuming that when you buy it there nothing will be wrong with it whatsoever, which IME is very unlikely.
If it's a cost saving exercise, you are likely to lose more money by buying one of those cars. If it's an upgrade you want, trust me, you are barely upgrading at all.