Whether a Neopolitan mastiff- pitbull mix is illegal in the United Kingdom depends on what the individual dog looks like. In the United Kingdom, any dog that looks like a pit bull terrier, a Japanese tosa, a dogo argentino, or a fila braziliero may be declared a "banned dog."
The decision as to whether a particular dog has sufficient traits to be banned will be made by police or council dog expert. The decision is made based entirely on the appearance of the dog. Mixes of mastiff and pitbull type dogs are called bandogs. Bandogs look somewhat similar to most of the dogs on the banned breed list.