The slipways round here are generally quite poor -- very tidal and mostly shingle.
There is nothing at Brighton marina although you could probably be craned in for an extortionate fee.
Lady Bee Marina at Shoreham has a decent slipway which has all tides access. You will have to lock out of the dock though which is a bit of added aggro (only operates once an hour and not at all at low water springs).
Otherwise there are a few lesser slipways around Shoreham up the river. Have a look at www.boatlaunch.co.uk
Another option would be Newhaven where I have launched (a long time ago) from the sea angling club. Not sure if it's still there, although there may be other options.
If you're coming from the Solent I would seriously consider not bothering to trail and doing it by sea. It will take you an hour or two and will probably be quicker than trailing in the long run!