Hello, welcome to the forum.
The electrical safety council have a website that can offer some unbiased consumer guidance to electrical related issues:-
e.g. Search RCD on their site brings up:-
https://www.electricalsafetyfirst.org.uk/search/?q=RCD&p=1
With answers such as:-
https://www.electricalsafetyfirst.org.uk/blog/why-you-need-an-rcd-fitted-in-your-home-asap/
Roll back 20+ years and RCD's were not required by UK wiring reglations..
then gradually over time various requirements have been implemented such as:-
RCD's for external wiring.
RCD's for sockets likely to supply external appiances.
RCD's recommend by some manufactures. (e.g. electric showers)
RCD's for sockets used by unskilled persons.
RCD's for cable buried in walls.
RCD's for all circuits in bathrooms.
RCD's for all circuits supplying luminaires (lights!).
Many older CU's had a single RCD or two RCD's protecting multiple MCB circuits..
Which means a single fault could remove power from all or half of your installation.
Whereas, individual RCD/RCBO's for each circuit will minimise nuisance tripping, and the inconvenience due to a single fault.
Basically, current wiring regulations now recommend that ALL wiring in a domestic installation has individual RCD/RCBO protection per circuit..
And whilst there is no requirement to upgrade older installations done to earlier issues of BS7671 wiring regulations.
Industry guidance and good practice recommend that ALL alterations/additions should comply with current wiring regulations,
and appropriate certificates issued with a signed declaration that the alterations you have done, do comply with current regulations.
Assuming the existing RCD is fully functional, I would have no issues with adding your additional socket.
But I would also add a covering letter recommending that the consumer unit is upgraded to a fully equipped RCBO board at your earliest convenience as this could reduce major inconvenience if you had an earth leakage fault whilst away on holiday, or at an obscure time of a weekend or bank holiday when you are unable to restore essential power to fridges / freezers / boilers / ovens etc..