More to the point, the limitation on 15 sq metres within 0.5m of the boundary does not apply for buildings made of 'substantially non-inflammable materials'. I am going to check, but assume that blockwork would be considered substantially non-inflammable.
I assume that this is to avoid a major fire risk to fences and neighbouring buildings.
I am not too concerned about high level security - the crime rate is quite low around here - I am mainly concerned with 'bangs per buck' and ease of construction plus conforming to any building restrictions. Being able to go over 15 sq metres within 0.5m of the boundary may be a strong argument for blockwork. The garden is fairly small and plan #1 has already been abandoned because the shed all the way across the bottom of the garden would be too intrusive, especially with the 1m gap from the rear fence.
One attraction of blocks is the low ongoing maintenance. Timber is attractive but does need regular treatment to avoid rot.
It may also be easier to hang shelves, benches etc. from a block wall.
This is now making me wonder if I will need pillars to strengthen the wall - or if a 5m run of single block will be O.K.
Nothing is easy ;-)