My medical hadn't lapsed, but was temporarily suspended.
When the new one was issued I was told on the phone by the CAA Doc at Gatwick, or rather by his secretary, that I was okay to fly whilst it was on its way to me (but not yet in my posession) as it had been stamped and signed by said Doc and that was what mattered.
Couldn't tell you if that advice was correct though.
WF.