I've done it. I took a SAW from Hong Kong with me. Like everybody says, make sure you declare it, both leaving your destination and arriving here, and in packed luggage.
To make things easier, have a local shop in UK put together a doc that says the fps using 0.2gBB, make and a picture of the airsoft in their company letter head. as long as it's shooting between 366fps and 500fps, you should be fine. also bring a copy of the CBSA Memororandom D19-13-2 pages 8-16, in case the officer isn't familiar with the allowance. One little thing that should help too, be humble and honest when you talk to the officer. It only took me an extra 15min to go thru the thing and they didn't even look at the packed airsoft. Good luck.