Only one of my three cats had an Avid chip. I contacted the Defra arrival facility at Manchester (I much preferred arriving into a smaller airport over Heathrow) and was able to confirm that their readers would be able to pick up all three chips, which they did without any difficulty.
The arrivals people also suggested I fax the completed documentation over to them a day or two before the flight and they checked it over and confirmed to me in an email that everything was in order, which really took a load off my mind.