Or do you actually get a standalone IMCR this way, which you can renew with an IMCR checkride, and allow the FAA IR to lapse?
Yes, you get a standalone IMCr.
Quite handy really, if the FAA IR has "lapsed" due to currency (no approaches etc. for 6 months) you could renew it yourself using the IMCr to shoot the approaches.
The IMCr expires in the normal way though and needs to be renewed either by IMCr renewal flight with CAA examiner
or FAA IPC with FAA instructor. In the case of the latter you need to reapply for the rating from the CAA (and send them another fee).
Regarding the FAA IR requirements for a minute: One that someone with 125hrs *may* fall short of is the X/C requirement - You need to have 50 hrs PIC cross country, where cross country is defined as
> 50nm - so Compton Abbas to Old Sarum doesn't count.