Unfortunately, the Violent Crime Reduction Bill makes it illegal to import Realistic Imitation Firearms without appropriate reason to be importing one. For Airsoft guns, you'll have to prove that you are an active airsofter, and the only way to do that is to take out membership with a airsoft playing site, and that usually takes time and money. There is a loop hole, that states if the primary colour is changed to to make it resemble an Unrealistic Imitation Firearm, or a so called '2-tone', the only limiting factor is that you have to be 18 or over to buy them, but it will have to be from a UK website, I think that is your only option at this point.
Unfortunately, to pour more bad news on you, the dirt cheap springer copy I think you've found isn't available here, BUUT, I have found an electric powered one
here, and they will charge an
extra £10 to custom 2-tone it, if your willing to pay a total of about £70 for it. But it will be illegal to try and paint it back or anything that will make it look like a real one again.
However, if you are under 18, I would advice against it explicitly! As an Airsofter myself, I know full too well about how people just assume these things are only toys. They can course harm if used improperly.