The costs of that extra year are five years tuition fees (5 x £9,000 = £45,000), the costs of living as a student for an extra year (Imperial estimates £12,000), the additional costs of living in London, and one extra year knocked off your earning power as a consultant (with a bronze clinical excellence award... maybe £120,000). Unless things have changed, at Warwick you will only pay one year of fees (£9,000) and be eligible for the Department of Health bursary (living expenses + mileage + hospital parking costs) to mitigate the cost of living. If you volunteer in halls as a Resident Tutor then you won't pay anything towards your bills or accommodation during the degree. I would estimate the cost of choosing King's over Warwick at maybe £200,000...? How keen are you to move on?
I had the same feeling about staying at Warwick after my BSc but there's plenty of time to move around afterwards. My training in the six years after Warwick have taken me to London, back to Warwick (!!), the US, and now Oxford. Off to Melbourne in 2017...
If you had a place on the King's GEC then I would have more sympathy with your wish to try somewhere new. Warwick as a medical student is a very different experience to being there as an undergraduate and I would make that my choice unless finances really are no object.
Good job on two offers!