There is a searchable database of professional complementary health therapists on CompTogether who are looking to swap treatments. You need to join the site (it's free) to read each therapist's full details (qualifications/ who they are looking to swap with etc), as they are password protected. There are lots of therapists registered in the London area.
Take a look:
[link=http://www.comptogether.co.uk]CompTogether[/link]
Hope this helps,
Tracey