Crosman Backpacker is very useful
short range tool if you can get hold of one - or make one from a 1377 or a 1322 with the various parts available from over here or from the States. Very discrete, very quiet. However, as I understand it, you can't legally in the UK convert a pistol into a rifle - the power of the converted pistol shouldn't go over 6 ft lb as the law still sees it as a pistol. The Backpacker can be legally modded to go above its standard power of around 8ft lb as it is designed and designated as a rifle.
Theres lots of info on the web about American users of 1377 or 1322 in standard trim using them for hunting. That is frowned on over here due to their lack of power.
Another alternative is to convert a 2250 Ratcatcher to compressed air- lots of info on the web for this - and use the Backpacker detachable stock. Considerably cheaper than other take down pneumatics, albeit not of the same quality. 2250 in CO2 is not consistent in power output in cold weather - although it is easily possible to mod them to get 10ft lb. OK for summer work, unreliable in winter.
You can detach the stock from the Benjamin Sheridan Blue Streak with just one bolt which makes it very discrete, fitting into a rucksack- but you can't easily fit a silencer and its not worth trying to fit a telescopic sight. Go for a peep sight instead. If you can live with these limitations and the pumping and that its .20, its a good tool putting out 10+ ft lb
Pump ups (in my opinion) help you become a better shot 'cos you know that a follow up shot is going to take a minute or so of pumping, so you make the first shot count!
Just remember that you need the land owners permission to shoot when out and about - firearms offences can be really expensive and liberty threatening