The animal must be accompanied by a customs form. I got mine through
The Animal Reception Centre at Heathrow (phone: 44-208-745-7894/5 or the link is
www.cityoflondon.gov.uk/arc)
I called them and they were very helpful.
The actual clearance of the animal took 4-5 hours so plan accordingly.
If all the paperwork is not complete, the animal will go into quarantine. this also applies if the 6 months is not up i.e. you can split the quarantine between North America and UK.