Rdv's answer isn't necessarily correct. The 'third party - any vehicle provision' used to be fairly standard with all insurance policies (whether or not they were 'fully comprehensive'). However, several companies have now removed this element of cover from all of their policies (including 'fully comprehensive').
If you've not got cover through a policy of your own, the matter will probably be dealt with by a fixed penalty of �200 and 6 penalty points. If the case was to be heard before a court, the maximum fine is �5000, plus 6 to 8 penalty points, and the possibility of an immediate ban.
As has been stated, it's also an offence to 'cause or permit' someone to drive a vehicle without insurance (as your girlfriend has done). The penalties faced by the person doing so are the same as for the driver.
Chris