Gregory, check the British law system - I've heard from some paintballers that you can't have an obejct that shoots projectiles with more then 135 fps or some ridiculosly (fugget 'bout spellin') low speed, or it will be qualified as a firearm. Check
http://www.arniesairsoft.co.uk for details.
I've been asking people around in airports, and all airline representitatives told me that they will have no problem transferring your airsoft gun, as long as you don't take it with you on board, and put it in the luggage (an obvious fact, but I was told that several times by different people... figured I'd better tell you, too!
).
Also, when you will be checking your luggage in, make sure you tell them that you have a TOY that looks like a gun, but has no way of shooting real bullets or being modified to shoot such ("not a replica - or they might ask for additional documents" - hint from some fellow russian airsofters).
Now, I'm saying that airlines will be cool and all - but here's a tricky part - customs. I'm rather ... how should i put this... dissapointed with russian customs for their ..umm.. way of handling any situation relaited to firearms. I belive you'd have to declare your gun on the way from Russia, and how you are going to do that - I have no clue(airsoft is one big mystery for russian laws). In fact, if you succeed or not - please make sure to post here what happened in details.
Then again, I don't have any information on British customs - but something tells me they won't be too happy to deal with a "mad russian who brought a firearm to our beautiful, glorious, peaceful and vast kingdom".
But if brits will be cool (by the way - Brit + cool = don't mix
), then again you'd have to deal with Russians on the way back. I mean, if you get lucky on getting your gun out, you would still have to proove to the russian customs (and law) that your 6mm is just a toy. On another hand, if you have already visited some criminalistic agencies (the ones that confirm that your gun is safe, toy, etc) beforehand, and have the documents on you - then you'll be just fine.
But mainly - arrive to the airport(s) earlier, and best luck to you!