Have you tried the Boat Scrubber at Haslar Marina in Portsmouth Harbour? If is only a "bottom" scrub you require it will take about 20 mins- 30 mins.
Contact is 023 9251 0567 0r www.boat-scrubber.com
Its like an up turned car wash. Won't do stern drives, but you are probably shafts!
I'm with Deleted User on this.
Visit the underwater car wash as suggested further down, or have it lifted. You'll need to change your anodes and check the undercarriage surely?
Lymington yacht haven will do a 30min lift out and scrub, for £80 on saturday morning. Book in advance, contxt Haven boatyard. They lift it out on slings, there's no drying platform. This is the first marina as you enter Lymington from the solent, turn left immediately after the 4knots sign and the knackered wooden wave shields
I am actually in Halsar but I have yet to hear of anyone who is absolutely delighted with the srubber machine. Maybe you have.
Lyminton seems to be favpurite at that price. I only thought of using a platform by resting down from the stern tubes forward where all the weight is. It would give longer to do jobs than a simple lift and scrub.