Every ne has their own mixture for soil. Yours should be very well draining because most of the time the tree will get lots of rain. You don't want soil that holds a lot of water. You probably want 1/3 each akadama, lava rock, and pumice, which you can probably order from somebody in the UK premixed. Donaff garden soil because it will retain too much water. In those rare spells in London when it doesn't rain every day you will have to water it when it dries out.
Don't believe the weather predictions. They say rain is coming to fool you into not watering. Even if it rains, check the tree.
For a comparison with London weather 140 years or so ago, read Jerome K. Jerome, Three Men in a Boat.
I still haven't heard back from the company that sold the Bonsai Tree for their advice, so that is pretty poor customer service.
At the moment, it is still indoors as I wanted to find out what they said about it, but I have been making sure that it gets plenty of daylight. I also pruned the massively oversized leaves back on the branch to where they were at least a similar size to a majority of the small leaves.
I found the following options for buying some ready mixed soil, and would really appreciate advice on which one people would recommend please: