Can a submain have spurs?

It would be allowed.

What are your respected loads?

I believe that the design process (due to the known loads) would actually mean the test results could pretty much be calculated.

Many farm installations employ overhead line distribution on poles with tap offs to various buildings along the route. It's a similar situation (also think of the typical street distribution network). Just ensure you calculate conductor sizing, volt drop and fault protection correctly. You might be well advised to consider future demand increases at the design stage.

I'll protect the cable (3 core 10mm) upstream with a 40A MCB (I've decided to change the consumer unit first and get rid of the Henleys) on the non-RCD side.

Workshop will have 20A + 6A circuits. I'm rather limited by 3% voltage drop here for the lights, though I know the lights (LED) are quite tolerant because they are coping with the voltage drop on the existing woefully under specced cable.

Annex will have 32A + 6A circuits. The voltage drop caused by the annex is insignificant - it's 7 times closer to the house, and in practice the maximum likely sustained load from it is about 10A.

I don't think you have a problem.

Do not forget that that the demand is not calculated by adding up the MCB/RCBOs rating.