If I send my Phantom for repair under warranty which is due to expire next month. And the repair process and shipping causes warranty to lapse, is there a reasonable amount of time a repair or replacement can be sent back if it turns out to be faulty?

The warranty is effective on the day of postmarked and not when they finish the work. So send it in ASAP and keep a copy proof ot ehs hipping date.

The repair has to be successful, if the warrantee has lapsed during the repair period then you should test it out really well within a few days of receiving it so that you can reject the repair rather than need to make a new claim.

It is going to depend a bit on where you bought it, but if it was in UK or Europe then your consumer rights give you a lot longer yet for manufacturing faults to be sorted free (5,6 or 2 years depending where) so you could always return it to the retailer, probably with a little argument.  Not sure what the consumer rights law in Guernsey gives you.