Ed,
I was in a sort of similar situation not so long ago.
You are not able to apply for internal postions as this route was blocked off at the last contract negotiations (from what I was told). So if you are on a new contract (after course 213/214 I think) you can only apply for external positions and these are harder to find than a needle in a hay stack.
I do know someone who works for the Met and they seem to be continously recruiting. Think you would only have to do a couple of years before applying to transfer to a force where you want to be.
ML