Both of my pets have the US standard microchips (despite asking them to do the ISO one for the cat, the dog already had his).
DEFRA had no issues with it (at Heathrow), and I've since been to 2 vets who've also been fine with it. However, I am considering getting an ISO one put in just so that I know a shelter/vet's office could read one of them if either of my pets were to go missing. I haven't done it yet, though, thanks to the concerns about having multiple microchips.
So I would think that as long as you can get the chip # put on the rabies certificate, you should be fine. You can always call DEFRA to be sure, though.