Dont know if this will help you, but when we were looking or a place to keep Topaz we approached our local Marina. The cost to keep her on a berth there would have been some £600 per year.
However, on discussing with the owners they have a lot of space in the Summer when all (or most) of the boats are back in the water. They were happy to negotiate for a RIB to be parked on a trailer, with easy access for me to collect and drop off, and are charging me £140 for the year.
No doubt the South Coast will be more expensive, but if you try to negotiate just storage of a RIB on a trailer and you do the rest (launch, recover, clean etc) they may be more flexible.