If you havent been before, as said above I would suggest going to a game and renting to make sure you like the hobby before spending £100+ on a gun! If you have already been I would say go for any of the G&G Combat machine line, always a safe bet for beginners.
The "Licence" is actually just a defence to say that you have a reason to own a RIF (Realistic Imitation Firearm) Ie being registered to the UKARA Database will get you a UKARA number which is a defence which all airsoft retailers recognise to be justification to buy a RIF, to do this you just have to attend 3 games in 2 months at your local site and as long as they are also registered they will sign you up. If you aren't registered to UKARA you can still buy an IF (Imitation firearm) which is just a 51% two-tone sprayed RIF, you also to be over 18 to buy either of these.