Any shop should be fine provided, even online/overseas. As previously stated its up to the store to make sure your legit.
If its overseas you also need to make sure the shop can attach some proof of this to get the weapon past customs otherwise they can kick up a big stink.
With regards to filming if its anywhere that the public may walk/see your best contacting your local police station and just let them know to make 100% sure that you won't end up with a firearms squad facing you if they get reports of you with guns
Im from Sheffield as well, the only problem is I don't have a licence yet, will probably be 3/4 months assuming I get into a skirmish sometime this month but that will probably be too late for you.
Thinking about it your best bet is to buy the gun, do the film and sell it after. That way you get the shots with the gun you want and will get a relatively decent cash back, especially seen as the gun will never be fired you should be able to sell it for practically the sale value...
Oh and It is not illegal for any film maker to own a RIF.
Its a separate exemption to the law, it makes no odds for the UKARA.
Again its a defence not a get out of jail free card but its not tied to the airsoft defence.
EDIT: I realise this is really unclear so I'll try and clarify it:
Defences for owning a RIF:
1) Airsoft - regular player etc UKARA licence etc
2) film makers.
3) Civil servants or something.
Therefore having a licence doesn't matter, if you can prove your a film maker then your good to go as long as the shop, customs (if applicable) and the police local to the filming site are happy your good to go.