1. There is no license.
2. Only adults can buy imitation firearms
3. Don't talk to the police about airsoft - its not in their immediate purview(to be knowledgeable on).
4. Do not buy a sniper rifle (as a first/second gun)
5. Play with a rental gun or,
5.1. Buy an Electric Automatic Gun (AEG), spend between £110 and £160, but not more than £200 including eye protection, batteries/charger. You do not need a scope for your first games; if indoors, buy an under £15 flashlight.
To take a long story short, out of the box airsoft sniper rifles are the same as the AEG, but much slower and arduous to use.
You don't need a good gun to enjoy airsoft, but you do need one which is not unnecessarily limited.
If you really do move slowly, you might want to branch out into DMR or sniper play for airsoft after a few months, but even if you were wheelchair bound, no one would recommend starting with a sniper rifle.
EDIT:
If you didn't have other concerns, i'd have not said "but not more than".