I didn't think they allowed you to use flash in the transport museum? due to the light making old faded things fade worse?
all you can do is increase the light coming through the lens (wider aperature, longer shutter) or upping the ISO.
But each has its problems, so its all about compromise. In this case I'd agree with Fremlin, and using a tripod* would be the best option.
*or a monopod, if you don't fancy carrying a tripod around
