I looked into it a bit when I first arrived but it was a virtual non-starter.
IIRC it would have to have been transferred into an unlocked RRSP (one you can make withdrawals from prior to retirement) which NATS would never agree to.
There is/was a company I found via Google which allegedly "specialised" in moving pensions from there to here, but I didn't take it that far. I bet their fees would make your eyes water!
I don't think there's much to be gained from it. When you retire, having a pension paid in £ when you're living over here wouldn't prove to be a hardship.
I don't think Nav Canada will let you buy in with your old NATS pension either.