Yes, I sent my tax check by registered mail. It tracked to JFK in New York. That was the end of it. The USPS said they never got it at JFK. Royal Mail said they gave it to USPS. Each said the other was responsible. Neither would pay any sort of compensation for the fees I'd paid to have it sent. It was a royal pain in the patootie to get the check re-issued. And that was a "normal" tax check. I'd hate to have to wade into the stimulus check pool on this.
There is, of course, the benefit/cost analysis route. If you deposit it to an account here, how much do you lose in fees and conversion to the pound? If it's not more than the FedEx charge, you might go that route. Unless it really needs to be in your USA account.
If you are going to send the actual check, use FedEx.