Konan by the sounds of it you're a fairly straight forward no nonsense driver if your tyres are lasting you that long. The longest I've ever had out of a tyre is 18k miles from a set of Michelins which were guaranteed for 40k (got some money back on that one!).
What do you see as a sensible mileage for tyres as most of us here can only dream of seeing 20k under the same set of tyres? £50-60 for any 205/50 tyre is damn cheap and if you stick to budget tyres you should easily hit 20k+ miles everytime with their harder compounds.
However if you're likely to do 20k miles in one year again I'd suggest that budget tyres are a seriously bad idea. Softer compounds will mean a new set of tyres every year but when you spend that much time behind the wheel surely it's worth it?!
If budget weren't such an issue I'd say Michelins (Premacy for instance) all the way as it will last a while and certainly offer more grip than budget but not the levels that a short life soft compound tyre like Yoko, Toyo or even hard driven F1's would give.
If the budget overrides and you don't drive too hard I'd suggest looking at Continental as they err on the side of long life and won't break the bank either if you look round hard enough.
Good luck!