If you were not allowed to do it, Mike would have made it very clear. As it is, there are defined rules on what permits token placement in London and train permits held is not part of them. London, ho!
London Ho indeed. I do vaguely remember a game where a locally trained company managed to token London but didn't subsequently merge, making the token, um, decorative...
Memory fails me on whether this was intentional, but I fear it may have been a case of falling foul of merging requirements.