This area (the old Lewisham West constituency) suffered one of the greatest changes in the last boundaries review - which only came into effect at the 2010 elections. People who care about these things (probably a minority of us, but...) have only just really got used to that. The Boundary Commission initial proposals (far from set in stone!) involve yet another change for us, just as radical. AFAICS their main driver for imposing this on us is that they started at the bottom (of the map of London) and found they could fit 3 MPs neatly into the borough of Bromley!
Another disadvantage of "Dulwich and Sydenham" would be that the area most of us know as Forest Hill would be lost in the middle. People in Sydenham might fancy the idea of being 'twinned' with Dulwich - but I bet they'd be seen as the poor relations!
Despite the problems of straddling the borders of Lewisham and Bromley, there's a reasonable level of demographic consistency across most of the current Lewisham West & Penge constituency. An alternative with minimal change would be just to add Crofton Park ward to the current constituency. This would put us in the range of acceptable population. It's likely (under any proposal) that the other 2 constituencies in Lewisham would then also include parts of neighbouring boroughs.
Is the Forest Hill Soc intending to make representations to the BC? What's the consensus of their members? Are any other local groups (apart from us 'usual suspects' in the political parties) intending to get involved?