There are many dedicated airsoft retailers in the UK, some focus on on-line sales and others have physical shops. They may also also sell surplus, airsoft specific, after market or reproduction military gear. Have a look on the member benefits tab on this website for a list of great retailers who will provide UKAPU members with discounts and offers.
The United Kingdom Airsoft Retailers Association (UKARA) is a body which represents airsoft retailers. They administer a database which you can be registered on once you have become a skirmish site member. UKARA retailers can then check this database when you want to buy a RIF (a Realistic Imitation Firearm), which upholds their legal obligation to ensure that they only sell RIFs to airsoft skirmishers. It is also common for UK players to purchase airsoft equipment from big shops in Hong Kong.