Again, I'm no expert, check with Defra but you don't have to wait one month after vaccinating to draw the blood. People generally wait, as Stoatula said, just to be sure the vaccine has had time to produce an adequate antibody titer. If your cat has had rabies vaccinations in the past, then the titer should be fine anyway and you'll be OK, if not, you have probably drawn too early and the titer levels will be too low. Maybe you could consider getting blood redrawn and sent off now if this was the cats first vaccination. Regardless of any of this, you have to wait 6 months after blood was drawn to travel, which would be end of march at the earliest.
As for the paperwork, make sure your vet clearly states that the microchip was inserted BEFORE the vaccination was given, as it was the same day.
Good luck - I hope it all works out for you!