I doubt the CAA will grant an extension, but good luck. If you can get it sorted in the next few months you have plenty of time. Have you considered other options? If you are confident that you will get a class one back, depending on your circumstances you might be able to get an alternative medical, FAA class 3 for example, and tick off your hour building in the states. The CPL and IR need only take 25 hours in the UK.