Was in Brighton today and decided to look at this road on leaving - ie: driving north.
At the start of the road it is signed NSL; after about 50m there is a single

on the left with a plate below it saying "ahead". About 250m further and temporary traffic lights signal the road works mentioned - and, after another 200m or so (ie: within the course of the roadworks) the

seems to start, continuing for some distance beyond the roadworks before reverting to NSL; there are regularly spaced repeaters along the way (no street lamps here - no houses either!).
It was dark and difficult to see the signage for southbound travellers but it appeared that the limit in that direction extends to the junction at the end of the road.
The signs do appear permanent and I would hazard a guess that the roadworks are to do with the limit rather than the other way around. As
davemar says, a stupid and unecessary limit. Can anything be done? Depends on the traffic order made - worth checking, it MAY be only a temporary restriction - on the other hand it could be an invalid order! A petition would be a time consuming pastime with little hope of achieving anything, beyond wasting some council weenie's time - which may not be an issue, but letters and/or phone calls could achieve the same effect for less effort.
I didn't think to look, but presumable the Ditchling Road continuing into Brighton is still NSL as far as the golf course entrance?