I've jumped through the PETS qualification hoops with my dogs.
In a nutshell:
*** The steps must be done in the order given - no exceptions.
(I've met people who were refused entry because the order was wrong.)
First, your cat should be microchipped with a UK readable chip - details on the DEFRA website. If another type of chip is used, you will need to provide a scanner. http://www.defra.gov.uk/animalh/quar...info/micro.htm
Once chipped, your cat needs to be vaccinated. http://www.defra.gov.uk/animalh/quar...info/vacci.htm
After vaccination, blood needs to be drawn to do a rabies titre test. This is when the clock starts ticking. http://www.defra.gov.uk/animalh/quar...info/blood.htm
It is usually recommended to wait 2-4 weeks after vaccination to do the test, as it takes some time for the antibodies to develop. If this is the first time that your cat has been vaccinated, you may not show sufficient antibodies if blood is drawn too early. However, if your cat has been regularly vac'd, chances are that he already has a fairly high antibody level. The waiting time between vac and blood draw is not specified by DEFRA.
(I know of one person who, faced with a shortened schedule, had her dog vac'd and then five minutes later did the blood draw.)
The results have to be interpreted by an lab recognized by DEFRA (here it will be the Tollwutzentral in Bern) - Your vet wil organize this for you, and you will receive an official form with your cat's titre levels. http://www.defra.gov.uk/animalh/quar...-info/labs.htm
(I have no idea what the cat range is - for dogs, DEFRA mandates a minimum level of 0.5 international units. To give you an idea, my guys showed titre levels ranging from 2.3 (youngster who had had puppy vacs and then 1 annual booster) to 14.7. (10 year old, regularly vac'd.)
Assuming you meet the minimum antibody level, you now have to wait 6 months (and not one minute less) from the date of the blood draw.
You will need a pet passport to record all info; your vet will give you this. I always keep copies of any supporting document with the passport. http://www.defra.gov.uk/animalh/quar.../documents.htm
Once you have met the waiting period, you are almost good to go.
Last step: no less that 24 and no more than 48 hours before entry to the UK you will have to treat your cat for ticks and tapeworm. http://www.defra.gov.uk/animalh/quar.../parasites.htm
You must use a recognized preparation - a substitute might not be accepted. (I know of someone who was refused because of this, and had to do a day's quarantine to be re-treated.)
If you are driving, be sure your travel time allows for the 24/48 rule. (We usually drive through France, spend a night somewhere nice, and have a vet at Calais do the tick/tapeworm treatment before taking the Tunnel. )
You must enter the UK via an authorized route. http://www.defra.gov.uk/animalh/quar...nfo/routes.htm
The important thing is to do everything exactly as listed on the DEFRA site, in the order specified.
My vets here have all been fairly conversant in the process - if yours isn't you might want to use someone who is familiar with PETS to help you.
Your passport is good as long as the vac is kept up to date according the the manufacturer's specifications. If you re-vac one day late, however, you have to start the process all over again.
For further advise and commiseration, see the UK-Yankee website (a forum for American expats in the UK). Lots of posters there are going through, or have successfully navigated, the process: http://talk.uk-yankee.com/
(Bear in mind though, coming from the EU/Switzerland is easier than from the US)
ETA: Oops, cross-posted with Smackerjack.