Do you mean for one at home or a school one?
There used to be the fame academy bursary, although I'm not quite sure how that works and I think there were boundaries on age etc. and what the instrument would be used for.

Seem to vaguely remember something else to do with the BBC but that might just be me! I think music centres sometimes hire out instruments although I suppose pianos would maybe be a bit unlikely?
sorry I'm not much help!!
xxx
P.S. have just looked up the fame academy bursary, it now appears to be the BBC performing arts fund and is open to 11-15 year olds, bursaries up to £1200 are avaliable, can't remember how much pianos cost but I would have thought that would get you a fair way there...
http://www.bbc.co.uk/performingartsfund/iande/