Rick wrote:

Do you have a link on custom cable sizes, where the cables are totally
in the insulation,

That really is a non-preferred way of doing it. The cable rating should
be taken as being half the clipped-direct rating. For a ring, where you
need 20 A as-installed rating, you'd need 40 A clipped-direct rating,
which would mean having to use 6 mm^2 cable. Since the terminal
capacity of most BS 1363 accessories is 10 mm^2 you have an immediate
problem in that it would be difficult to fit the two ring cables in the
terminal, let alone any spurs. IOW forget it.

or can I simply use 2.5mm cable, and reduce the MCB
size. If I have a ring for each room, this would be fine by me, most
rooms will be bedrooms, studies, only the kitchen has lots of power
requirment.

I'd advise strongly against such a non-standard design approach for
power circuits. There may be circumstances where you can get away with
cables totally embedded in insulation, e.g. parts of lighting circuits,
but you still need to be careful - e.g. you may need 2.5 mm^2 cable on a
10 A lighting circuit and could run into problems with terminal sizes again.

--
Andy