Shoreham cement works would be your best bet, its an awesome place and pretty vast but there can be issues with security
here's a couple other places in Brighton I can think of off top of my head maybe worth a try;
The old barracks in Brighton, On Lewes Rd (near B&q Halfords) Theres 2 derelict buildings ones quite heavily graffiti'd. theres also a decaying old truck diesal pump a small industrial estate loads of piles of rubbish and flytipping and some waste land. You used to be able to get into one of the derelict buildings if you have a look around the back! (If you plan on going in Take a torch and a friend its pretty dark and grimey in there)
Theres an old trade fruit/veg market by what used to be the ocean rooms which is pretty cool, I think its still in use but its quite industrial and starting to decay, its behind the grande parade not far from royal pavilion...
Black rock (near the marina) Used to have some interesting bits of old seaside architecture, buildings and a huge legal graffiti wall but I Know they were starting to develop it down there so it may all be gone..
There's also some interesting buildings around London rd which are prob worth a check, behind lloyds banks a good place to start
And Shoreham power station may be worth a look its very industrial out there, old and new buildings, most the new stuff you cant get that close too but may be worth a visit anyway
most the other places I know have either been bulldozed or are being developed but hope that helps for now
if you need directions to any of them spots let me know
and if I think of anymore I'll let you know
