Will using this switch to control a relay which in turn
controls my lighting circuit mean that I can then use the
CFLs, as they are not directly controlled by the time
switch?

Yes.

If so, then
(a) can I run the switch == relay circuit and the
relay-controlled lighting circuit from the same
6A MCB/RCB0 in the CU?

Yes.

(b) can anyone recommend a suitable relay
(with a web link if possible)

Personally, I'd be inclined to fit a DIN rail contactor. It can share an
enclosure with the external RCD. A cheap "shower" or "garage" consumer unit
with RCD incomer could be used, and the MCBs thrown away. Not the cheapest
solution by any means, though.

Christian.