Upvote:1
It is generally a good idea to apply for visas backwards.
If you already have a double-entry Schengen visa which allows you to go on the continental part of the tour, you don't have to supply too many details of that to the UK -- they can see that a Schengen state has looked into it.
The UK will also be reassured that you can legally go to the Schengen states, which makes it appear less likely that you'd overstay in the UK instead.