I don't think SERA applies because the UK CAA have seen fit to exempt us from many of its Rules, to our benefit, it seems.
As I understand it, the semicircular rule only applies above 3,000' (not forgetting that the lower flight levels aren't available in times of high atmospheric pressure).
Let's take a worked example. You're flying between Bovingdon and Brookmans Park VORs. The cloudbase is around 1,000'. You decide to fly IMC above the MSA which I think is 2,200', so you choose 2,400' to remain 100' below class 'A'. You obtain a Traffic Service from Farnborough. You have a valid IMC rating or equivalent. Can't see what isn't permitted there...
TOO