I'll chip into this as well, i also did such a thing back in 2008 august.
I am just gonna add in ones that krisblade missed out, or have conflicting information:
Missed:
KCL - We would never advise potential applicants to start a degree with the intention of leaving it, it does not look very positive on a UCAS application and to apply to medicine you need as competitive application as possible. Subsequently if you are considering medicine then you should either complete your degree and apply as a graduate to our 4 year programme or take a gap year and apply from September for 2009 entry.
UCL - I am afraid that it is not the policy of UCL Medical School to consider students who have failed to complete a course for transfer or for admission to the first year of the Medical degree programme at UCL. Candidates currently holding offers elsewhere or who take up a place through Clearing should be aware that if they are formally placed on a degree programme, they will be expected to complete it, obtaining at least a 2.1 in their degree, and will not be eligible for application to this School until their last year of study. Additionally, although the School expects that some candidates will also apply for non-medical degree programmes due to the pressure on places, sometimes with a view to applying as a graduate, we would not look favourably on any candidate who, once placed on another degree programme, decides to reapply for medicine in the following year and does not withdraw their application via UCAS. Failing to take up a place on another programme deprives another more committed candidate of a place and wastes the time and effort of admissions staff.
Liverpool - We would not consider you whilst you were on another degree programme.
HYMS - Thank you for your email and your interest in HYMS. If you start the degree at another university you must complete it before applying to medical school (you can apply in your final year of the course if you are predicted a 2:1).
Conflicting information:
Newcastle - You could submit an application to study medicine at Newcastle, but if you decide to start another degree the University would advise you to continue and finish the degree then apply as a graduate with your degree classification as your entry requirement.
If you have no intention of completing the degree, Newcastle would advise you not to start the degree, apply using your A levels as your entry qualification and use the next year as a gap year.
Glasgow - If you chose to commence another degree programme other than medicine you can apply through UCAS and have your application considered. However, please note that priority will be given to applicants who are school leavers, graduates or completing access courses.