rainwater into soil pipe??

NO you can not. But who is going to police it. Only If you are working with BCO or want to sell the house will it be picked up. But if I seen it on a job. I say look at that BODGE JOB JMc

 

Can you not move your downpipe a foot or so along .Quite straightforward and cheap.
Use a 45 degree bend to direct the water into the existing gully

 

You are allowed to drain your roof into an underground sewer provided area drained is less than 40 sq metres.
I have never seen what you are proposing and it would look as Jmc said very unprofessional.
I can't see how you would be any better off as you would then have a hole in your conservatory roof.
Why not just box them both in a false wall.

 

It is not prohibited from my quick check of British Standards and Building Regs, in the old BS5572 (Code of Practice for Sanitary Pipework), it specifically mentions the admission of rainwater into discharge stacks. Rules were max roof area 40m2 where there is a combined drainage system.

However, it is undesirable, just think if the base of the stack blocked and during a rainy day the water built up in the stack until it found the easiest way out, probably from your WC pan. Your call.