And 6mm from the gridswitch to the sockets? If you've dropped to
2.5mm from the gridswitch to socket that section is not properly
protected by the 32A breaker...

Yes it is. It is protected from short circuit within the cable by having
sufficiently low earth loop impedence. It is protected from overload by the
13A plug top fuse. It is no different from having unfused 2.5mm spurs from a
ring main. You are permitted to assume that a single socket will only draw
13A and a double socket 20A, apparently, but must still provide short
circuit protection, which is easy enough with a B32A MCB if the 2.5mm
section is short.

Christian.

P.S. the 2.5mm actually runs from before the grid switch, as there is no way
you can get two 6mm cables in one terminal.

2.5mm 2.5mm
6mm #--------SW-----SKT
=======###-------SW-----SKT
#--------SW-----SKT

## = junction box
SW = 20A DP switch
SKT = unswitched 13A single socket

The 2.5mm sections are kept short to reduce earth loop impedence at SKT to
ensure that 160A can be reliably drawn on an earth fault at the furthest
point.

Christian.