John Rumm wrote:

An alternative is to use radials, not rings. Then any additional sockets
required can be spurred off existing socket locations, without having to
do down-along-up to preserve continuity of a ring.

I did think of that, but you don't gain much in this case since I don't
think you can get 4mm^2 cable down the conduit either, you would still
be limited to 2.5mm^2 conduit drops. So extension would still have to be
via a new drop rather than from an existing socket.

Doesn't have to be 4mm^2, you can wire radials with 2.5mm^2 and use a
20amp MCB. This is what I have done for a lot of new circuits to the
study and places like that in our house. It's much easier in many
ways than a ring.

--
Chris Green