I'm probably coming at it from the other end to Mikefule. I've dealt with rescue boats on motor yachts and the requirements were not that onerous. In addition the surveyors tended to apply their own interpretation so sufficient space to lie a casualty down became sufficient space to sort of let a casualty slump. Other than this requirement that defined the lay out of the boat most of the other requirements were met simply by buying a half decent SIB / RIB and kitting it out with basic safety gear.
I think if you refer to them as safety boats rather than rescue boats, it will probably smooth the path somewhat.
The outfit below have a few safety boats on their books and they don't look overly complex.
https://www.livetts.co.uk/fleet/
On the assumption that you are not a competitor to Livetts, you could give them a call and ask how their safety boats are certificated.
Alternatively ring your local MCGA office (Liverpool??) and ask for some guidance.
There will be some crew training required but if it is to 'simply' retrieve someone from the water if they fall in, it might not be too onerous.