Sorry man, but you're pretty much SOL with bringing it with you unless you can meet some pretty specific requirements.
I'd say it's better to sell while in UK and buy back in Canada, however that said Canadian stuff is quite a bit more expensive than stuff outside of Canada (namely US and Asian pricing).
One thing that you could do however is strip apart your rifle and take everything EXCEPT the prohibited parts, which is basically the lower receiver on the M4 and then buying a new lower once in Canada. You're also in luck that it's a G&P because then you can contact a retailer who does custom orders once you're in Canada and AV'ed and you can get a kickass G&P custom lower.
The pistol I'm sorry to say if you do the same thing you'll have to ditch the lower frame and it's not that common that the AV retailers import specific frames (although I do believe there have been some that get the odd "upgraded"/steel pistol frame once in a while). So really it's probably in your best interest to sell the pistol.
PS: As for the UKARA thing, we don't have anything like that at all. The closest thing is the Age Verification program that we as a community have self implemented and is in no way backed by any governmental body (whereas the UKARA Skirmishers Defence thing is recognized as a legal defence around the VCRA regulations).
EDIT: Be warned that on occasion you may get some overzealous CBSA officer who seizes your gearbox as a "lower receiver" however the remedy for that is to appeal and get the firearms expert to basically say "WTF is this on my desk? This isn't a lower!" and release it to you.
ಠ_ಠLess QQ more Pew Pew
READY TO >> RACE
Last edited by L473ncy; January 5th, 2011 at 14:50..