It sounds like a great idea particularly if you're confident that you will get along together. As someone said, it seems to be becoming very popular and practical these days, and in later years if you need help, it could save you moving into care.
Would it be possible to have a trial period before you make any definite decision and selling your own property?
I'm sure there would need to be certain legal conditions in place so that your situation would be secure.
Good luck with whatever you decide, please let us know, but I feel you will make the move.
To think that London, or anywhere else for that matter, does not belong to any one demographic