Fugi, as an under 18 it is a CRIME for you to buy an airsoft gun, regardless of what colour it is. Someone who is over 18 will have to buy it for you, if you want it to be in realistic colours (e.g. not bright orange) the person buying the gun must be able to provide the seller with a defence against prosecution under the VCRA, for the purposes of importing the best way to do that is by being registered with UKARA. There are no under 18s on the UKARA database. If you want a realistic looking gun you'll need to get a responsible adult to go with you (and play!) to 3 skirmishes in NOT LESS than 2 months and NOT MORE than 12 months, they'll then be entitled to have their details put on the UKARA database. That person can then buy a RIF (realistic imitation firearm) and safely import it.
YOU CANNOT BUY A GUN. DO NOT TRY, IT WILL BE DESTROYED BY CUSTOMS AND THEY DON'T DO REFUNDS.