Possible to move external output pipe for Shower?

Hi,

I am a novice DIYer so apologies if I don't make perfect sense.

I live on the first floor of a 4 story Georgian apartment building in London (on top of the ground floor apartment) and am wanting to replace the bath with a walk in shower. I would like the shower tray to sit as flush to the ground as possible, which will mean dropping the pipes down approx 12 - 13cm from the current level. In addition to the internal work, this will also require creating a new hole in the wall for the external output pipe.

My query is around dropping the level of of the external hole as this will have a direct affect on the roofing of a glass conservatory type structure belonging to the ground neighbour.

I have attached photos of the glass roof and the current position of the external output port for the shower. The glass roof is sealed with a strip of lead and dropping the level of the pipe by anything up to 13cm would mean:
1. cutting into the lead
2. lowering the bottom level of the pipe to approx 5cm above the glass line from the current level of approx 18cm

Before I discuss and agree anything with the neighbour, I'd like to know first if this all sounds physically possible and within UK building regulations.

Any advice much appreciated!

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.