The nearest mainline rail station is West Drayton, which can be reached directly from London (Paddington) or from the west-country (eg. from Bristol). Then take a taxi or see "By Bus" for instructions to get from West Drayton to Brunel.

By London Transport Underground:
The nearest underground station is Uxbridge on both the Metropolitan Line and the Piccadilly Line. Then take a taxi, or see "By Bus" instructions from Uxbridge to Brunel.

A large map of the London Underground can be downloaded here.
(Uxbridge is in the top left-hand corner).