I brought my cat from Canada. It is really easy, but you have to do quite a bit of legwork. Also, you'll have to start at least 2 months in advance from when you expect to arrive in Hong Kong, or your cat may have to be placed in quarantine.
You shouldn't need to hire a company to do it for you. Just follow the instructions starting
here. There is some also some information on the above site.
Most importantly, make sure your airline can handle carrying your pet, and know what they are doing. I highly recommend Cathay for this, as every other airline I checked into either did not carry pets, or gave the 'run around' so I didn't have any confidence in trusting them with my cat.
If you have a LOT to do, and don't mind the extra cost, it may be worthwhile to hire another company. honestly I don't know what that export-a-pet company would charge. If it's not much, it might be worthwhile afterall.
what's the world coming to. does the cat get a rub down from immigration officials too 
i had my cat flown from the us to hong kong and then to italy a year later, and export-a-pet handled everything for me. just make sure your cat has his/her shots done... something about no more or no less than 30 days of departure or something like that... also, i think some airlines require that your cat has a microchip... why don't you call up export-a-pet or email them, they will give you all the info that you need. they've helped me out a lot, even in tough situtations where my cat was being held ransom at the italian quarantine. oh and for long haul flights they'll even put your cat in a 'pet hotel' for some rest and excercise 
i'm sure if your cat is coming in from england then there shouldn't be any quarantine. oh and umm don't forget to line the kennel with one of those diaper pads, get a hamster/rabbit water bottle, etc...
