As on British Airways operated flights, recognised service dogs are allowed in the cabin, as well as other pets up to 6kg. Other pets must be transported in a waterproof bag or cage, with maximum dimensions of 20 x 30 x 20cm (7.5 x 11.5 x 7.5in).
Does Heathrow allow pets?
You will need to ensure that your pet is at least 15 weeks old before bringing it to Heathrow and if it's a dog, ensure that it has been treated for tapeworm. Whilst at the airport, you will not be able to bring your animals into the terminals, unless they are assistance dogs.