With reference to wikipedia the law as it stands on the legality of airsoft guns as of 2017 is:
"According to Section 36 of the VCRA (Violent Crime Reduction Act 2006) which came into effect on 1 October 2007, RIF's (Realistic Imitation Firearms) may not be sold, imported, or manufactured. Unrealistic imitation firearms (IF's) must have their principle color as transparent, bright red, bright orange, bright yellow, bright blue, bright green, bright pink, or bright purple or have dimensions of no more than a height of 38 millimetres and a length of 70 millimetres (as defined in the Home Office regulations for the VCRA). Exceptions to the act are available for the following:
a museum or gallery
theatrical performances and rehearsals of such performances
the production of films and television programs
the organisation and holding of historical re-enactments crown servants.
The notes for the VCRA state the following: "The regulations provide for two new defenses. The first is for the organisation and holding of airsoft skirmishing. This is defined by reference to "permitted activities" and the defence applies only where third party liability insurance is held in respect of the activities." and "The defence for airsoft skirmishing can apply to individual players because their purchase of realistic imitation firearms for this purpose is considered part of the "holding" of a skirmishing event."
It is best to check the latest laws to see the legality of purchasing airsoft guns for playing airsoft sports.