There are items of interest to Thames Barge enthusiasts in various museum locations throughout the country. I'll try to list those I discover, and would be grateful for feedback about other locations that can be included.

Dolphin Yard Sailing Barge Museum, Sittingbourne, Kent - a museum about all aspects of sailing barges, housed in the buildings once used for repair work. (note this museum is currently closed, awaiting relocation.

National Maritime Museum - some of their collection may be on display from time to time. Also hold important collections of models, plans, films and photographs
Rochester Guildhall Museum - a variety of barge related items on display
Medway Heritage Centre, St Mary's Cottage, Dock Road, Chatham, Kent (Tel 01634 408437)
Mersyside Maritime Museum, Liverpool, has a Model of a Thames Barge in a bottle
Lowestoft Maritime Museum - some barge models amongst the impressive collection of models of local fishing boats
Swansea Museum has a model sailing barge
Twickenham Museum - interest in barges in the upper reaches of the River Thames
Whitstable Museum - some barge models and other items refecting the local importance of sailing barges on the town's history