Or you can apply for if you do not already have one a US ESTA fly to Seattle cross over the land border to Vancouver with just your COPR and passport. Once in Canada you can fly direct back to the UK no need to go back through the US although guess the flight ticket cost might be the decider. People waiting for their PR card either new or having an expired one often use this as an alternative rather than wait out a PRTD. For the latter really depends how long you have to wait it out in the UK , not sure for UK but possibly without your passport.