It looks to me that the outside elbow could removed and replaced with that existing "Tee" on top. The line is pitched toward the right in the picture. The cleanout could be inline with the pipe and the tee tap could go directly inside, at the new lowered level, to connect with the shower drain trap. The lead flashing presently is embedded into the morter, so when you lower the hole position, you will need to groove out the brick around the pipe so that the flashing edge can be regrouted in.
Since I have no knowlege of UK plumbing specs, this may or may not be acceptable.