Orient Express - Cheap Chinese Rims / Wheels / Forks / Frames / Cranks / Etc

20 quid for a carbon areo seat post, are they any good?

You do the math. I'd take the claimed rrp with a huge pinch of salt, it's a no-brand knock off of the Alpina you can buy from Dolan for £60

would it be ok to use a spacer to shim up to my required 31.6?

Shimming up from 27.2 to 31.6 isn't a problem, but using a 31.6 post would still be a better plan.