Four Double Sockets on a 10A Radial.

[JohnW2]

Yes I'm not sure whether the OP has made things worse by running the cable in conduit.

I find the regs all rather confusing. 2.5mm² cable 'totally buried in insulation' (Method 103) has a CCC of only 13.5A - which is the figure the OP feels he has to use. However, with Method A (described simply as "enclosed in conduit in an insulated wall"), the CCC would be 20A (per Table 4D5). However, as I've said, one wonder whether Method A is actually intended to encompass all situations of 'cable in conduit in an insulated wall' since the diagram for Method No. 2 (which is Reference Method A) in Table 4A2 shows the conduit in contact with the inner leaf of the wall (although the description does not specify it as a requirement) ... so I just don't know for sure!

or ... whether electricians don't work to the letter of the requirements when insulation is involved,

Indeed. As I've said, I strongly suspect that some aspects of de-rating are often 'overlooked', deliberately or otherwise. If that were not the case, then I strongly suspect that 2.5mm² cable would not be regarded as big enough for many a ring or radial final!

... buts its a sad day when you can't have some sockets on a 16A MCB with 2.5mm cable

Quite so - and I'm very tempted to say "unnecessarily sad".

Kind Regards, John