A person will qualify for a multiple visitor in transit visa as long as their passport and visa for the country of destination are still valid for at least two months after the expiry of the multiple entry visa. This will allow the applicant to transit the UK any number of times within the period for which the visa is valid. The two month rule for the multiple entry visa is not compulsory for a single entry visa.
Fonte : Agência de Fronteiras do Reino Unido , p.12