Hi Nick,
I brought last yearto the UK my adopted mutt (boxer mix) with all the fears that they would think she had some pit bull in her. In the US people used to ask me if she was a pit bull, so that raised my concerns.
However, her vet said that eventhough Sasha could have some pit bull in her bloodline, she was mostly a boxer (body type, underbite, etc). All her paperwork said she was a boxer mix, and I attached also paperwork from the adoption center which stated the same thing.
She cleared customs without any problem, and (sadly and not recommended at all) spent some time in quarantine. Now she is doing fine and living a wonderful life. Sometimes there is people that look at her with some fear, but that's something that we will have to live with. As long as I can keep out of trouble, I don't think that's ever going to be an issue.
I don't know if you have tried this document from DEFRA's website, which has pictures and also have description of the main characteristics of each prohibited dog :
http://www.defra.gov.uk/animalh/welfare/domestic/ddogsleaflet.pdf [nofollow] If only I knew how to attach pictures here, I would send you a picture of my dog, so you can't judge how much PB she looks like, and you could compare with your dog.
Hope this helps and good luck!