Just a brief (for me) reply - depends on what your expectations are. As a student at a foreign school designed for foreign residents (as opposed to schools designed for Americans - whether in the US or not) you should not expect to hear your professors say "you will need to know this for Step 1" nor should you expect that the library will be stocked with USMLE review materials.
However, many of your faculty in London will have done fellowships in the US and are familiar with the NBME procedures. And I'm sure that the curriculum there will provide you with the basics you need to do well on the USMLEs. The major factor, regardless of where you go, depends on you. You can order review books or bring them with you, and some London bookstores do carry the more popular ones.
I agree with Stephen that studying in London would be much more interesting FOR ME - but I caught a LOT (a LOT) of flak for my decisions and you must be aware that whatever your decision there will be people ready to criticize it (ie, "why didn't you go to an allopathic/US school? You must have been rejected from all of them, etc.") In addition, I am not sure that all PDs see an education outside of the US (even in the UK) as preferable to the US DO. Fortunately, there are many who will find you an intruiging, "out of the box" candidate by doing so - AS LONG AS YOU DO WELL. Do not expect that your interesting application will replace good scores on the USMLE or having some 4th year US clerkship experience.
Also see what financial options you will have - London is notoriously expensive - will you be eligible for enough loans (or have you enough savings) to see you through your education?
I know what I would do in your situation and I made the right decision for me. But only you can do the same for yourself. For further information on what steps will be required to obtain a residency in the US, please see :
http://www.ecfmg.org. In addition, inquire as to how much time you can spend in the US during your final year doing electives - this is important because some programs demand US/Canadian hospital experience and for obtaining US letters of recommendation.
Hope this helps.
