The Animal Reception Centre at heathrow is 44-208-745-7894/5 or the link is
www.cityoflondon.gov.uk/arcThey have an agent handle the animal and will forward you to the agent. but they are extremely helpful at HARC.
I didn't hire an agent because I had my brother in Canada to do it all for me (cat was 2 months behind us) but if I hadn't had a family member, I would have hired an agent.
I sent her through Canada - air Canada and there were no problems at all. I''ve moved this cat around a lot and always sent her in the cargo hold (travelling with us and without) and never had any problems
As far as I know, the customs form was a customs form to declare the animal. talk to the people at HARC, they'll make sure you have the right form.
I looked over all the paperwork again and really, there were no more charges. Make absolutely sure that all your paperwork is in order though and you will have no problems. If your paperwork is not in order, you will face charges because the cat will immediately go into quarantine. This is not at all the kind of government nightmare that you tend to see in North America, I certainly expected it would be and was more than pleasantly surprised.
I was very worried about the whole process also and in the end, it was quite easy, so keep your questions coming, I'm happy to help.