It's common practice for the Land Registry to record house transfer prices on the title register.
They don't record things like transfers if equity (did you pay her anything that was recorded in the transfer deed?).
On property sites it gives the last market related price paid so people can see when properties were sold to others (how the market is moving there ie does everyone leave so it may be a funny area) and for what price.
The more realistic price is what you bought for rather than what you transferred it to your girlfriend for. That's what I'd want to see if I were looking up local property prices.
If you did pay a significant amount then it is a remote possibility that the title at the Land Registry hasn't been updated as yet and you can check that here for �300...
http://www.landregisteronline.gov.uk/