Who lives where (or when) is irrelevant.
A dogs is a 'banned type' if it
looks like any of the following breeds:
Pit Bull Terrier
Japanese Tosa
Dogo Argentino
Fila Braziliero
i.e. it doesn't actually have to be a Pit Bull Terrier to be a 'banned type'; simply looking like one is good enough evidence for legal action against you and the destruction of your dog. Further, if such action does occur, there's effectively a 'presumption of guilt' since the law states
"If in any proceedings it is alleged by the prosecution that a dog is one to which section 1 or an order under section 2 above applies it shall be presumed that it is such a dog unless the contrary is shown by the accused by such evidence as the court considers sufficient . . ."
The power to enter properties (with a warrant) to search for and confiscate banned dogs is written into the law as follows:
"If a justice of the peace is satisfied by information on oath, or in Scotland a justice of the peace or sheriff is satisfied by evidence on oath, that there are reasonable grounds for believing—
(a)that an offence under any provision of this Act or of an order under section 2 above is being or has been committed; or
(b)that evidence of the commission of any such offence is to be found,
on any premises he may issue a warrant authorising a constable to enter those premises (using such force as is reasonably necessary) and to search them and seize any dog or other thing found there which is evidence of the commission of such an offence"
Due to the 'presumption of guilt' I've referred to above, I strongly recommend getting a vet (or other expert) to certify (in writing) that your dog is a Staffordshire Bull Terrier (or, at the very least, that it is not a 'banned type') as you may well need to produce such evidence (either to a police officer or to a court).
References:
http://www.legislation.gov.uk/ukpga/1991/65
and
https://www.gov.uk/control-dog-public/banned-dogs
Chris