At your budget I'd be looking for something nicer second hand that rougher but newer

I'd avoid CO2, I'm sure I will upset some people when I say I regard them as for fun plinking and not dispatching vermin, even though I know they can do it, but it's the way I roll

Your distances are not high, so you don't need mega power or accuracy, and although I'm 100% a .177 kind of guy I'd potentially be looking at .22 for your intended use.

I'd definitely look at getting an air cylinder - your not getting any younger by the sounds of it, and making it easier for yourself is not a crime, in the long term you would probably get one eventually anyway

There are plenty of us fairly local to you, and there is an excellent outdoor shooting club at: http://www.airgunshooter.co.uk/page4.html - and a few of us are members there, so it's something to consider if you don't just want to shoot rats and fancy a bit of banter and a coffee

Above all, do what your doing now, ask for advice if your not 100% sure, and with a bit of care you will get a decent gun sorted

James