As given by Dan Rodlie, the tug depict on this stamp sailing under the Tower Bridge in London is the LUCKY XI.
The image can be found on the Net as a model made by Revelle.
Not any tug under that name exists and it is believed she is made after a tug used by the USA Army of the ST type 257.
Which is depict is unclear but this type tug was built during World War II on 46 shipyards in the USA, she were built with a steel or wooden hull.
A total of 718 where built either as a large tug (LTs) or small tugs (STs).
The tug depict is one of the smaller type.
Sao Tome e Principe 2008 Db95000 sg?, scott?
Source: various websites.