The VCRA (Violent Crimes Reduction Act) makes it illegal to own a RIF (Realistic Imitation Firearm) without a form of valid defence (i.e. a reason to own one), there are a number of different forms of defence from Theatre and Film/TV use, Museums, etc. Airsoft has also been allowed as a valid form of defence.
The easiest and simplest way that UK Airsoft retailers decide whether or not your Airsoft defence is valid is via a thing called a UKARA number (UKARA is the United Kindom Airsoft Retailers Association), basically. If you go to a UKARA registered airsoft site 3 times over a period of not less than 56 days (I believe it is), and as long as you remember to register with that site they will confirm that you are actually partaking in the sport and therefore you be giving an identification number which most retailers will ask you for in order to sell you RIFs.
If you have another form of defence (all the forms can be found by searching the piece of legislation on UK Govt sites) many retailers will accept that too.
However, as long as you are over the age of 18 you can purchase an IF (Imitation Firearm) i.e. a two-tone gun. Some sites will charge you for spraying a RIF two-tone but many do this as a free service.
So, the majorities advice here and on other forums is to hire guns when you go to play for your first three games then once you are eligible purchase yourself a RIF. However if you simply cannot wait to have your own then you will have to buy two-tone. Now, as for turning that two-tone gun into a RIf once you have a UKARA number I am not sure as to the legalities of that. I'm sure someone else can help you with that, but I generally think that it is not a good idea.
Hope that helps.