From what I remember there isn't a slipway at Preston Marina!!
We used to keep our RIB at Fleetwood, the marina is more accessible than Preston, but there is a slipway on the western side of the river Lune (pretty much opposite the marina and ferry dock), which I think Dave Mallet (Wavelength on here) uses regularly.
The tidal range on the Ribble is quite big - and there is a sh*t load of mud wherever you try to launch... but I think the slipway I mentioned above is your best bet.
Otherwise, a lot of the ex-Windermere speedsters have migrated along the North Wales coast.........