I used to be quite into air rifles many years ago but with less and less places now to shoot i sort of moved away from them , but still own a BSA sport model.
As for 22 verses 177 the argument has gone on for many years about the for and against between them , but basically the 22 fans argue it delivers more of a whack due to the size and weight of the pellet where the 177 penetrates the prey due to its smaller size and faster velocity and is less prone to wind resistance .I have owned both and personally was in the 177 fan club due to the above mentioned.
Spring verses gas , again I'm old school and would go for spring , purely because i know how to strip and maintain them but gas i know very little apart from they are practically silent and have no recoil .
You would get a good 2nd hand spring BSA or even a Weihrach (great guns) for your budget
There is a member on here that has an amazing collection of Air rifles and will be able to give you much better advice than me , hopefully he will see this thread and offer better advice than i can give.
As for licence , you don't need one in England but i thin you may do in Scotland .
If buying 2nd hand and its been tuned , insure the muzzle velocity isn't over the legal limit or it then becomes a firearm