There is, as far as I know, no Tendai sangha in London. I attend a small, private sangha which is, as far as I know, the only one in the whole of the UK. As we are small, and we meet in the private residence of our teacher, we have very little fascilities and can't offer any kind of correspondence teaching, nor retreats. The next best thing would be to speak to Shomon, who is a Tendai priest in Denmark. She holds retreats twice a year and has an on-line course http://www.tendai.eu/
It is sad to say, that if you are looking for a close teacher-student relationship, Tendai might not be the best thing for you at this moment in time.
Hopefully, as Tendai grows in the UK more and more sanghas will open up.
The biggy would be for you to train in America http://www.tendai.org/
and become a priest
As you have noted, there is precious little in the UK and the west at large with regards to Tendai. This forum holds links to various books, websites and blogs that can help you.
There is a Pureland sangha in London with a resident teacher http://www.threewheels.co.uk/index.php? ... 6&Itemid=9
I have not been there myself, but have heard that it's highly recommended
Personally, it has taken me roughly 7 years to find a teacher (although, I was never really "searching" for one). So, these things do take time. If Tendai is what you are interested in, people like Jikan (on this board), Shomon and myself can give you pointers.
I hope you find what you're looking for,