Firstly, its £2.50 to open the barrier isn't it?
Secondly, Biggles how on earth did you work out the figure of £8? Do you close the gate when you are actually on the slipway???v
Thirdly, the fishermen have a swipe card thingy that opens the gate. You will notice on the pole where you put the money in there is a grey square; if you have one of these thingies and swipe it over that the barrier will unlock. I used to have one of these, you can buy them either from Portsmouth Sailing club if you are a member or from Ken Brown's office. I think it was £52 a year last year. Surly if you can afford to use a rib several times a week £2 or possibly £4 to open the gate on the rare occasion that the fishermen have closed it isn't a huge issue.
Fourthly, I use this slipway several times a week (I live just round the corner) and hardly ever pay. I really don't know what you're moaning about - it's a great facility, it's not in a bad state of repair, the poles and gate at the top are there to stop chavs parking on it, and if you know how to reverse don't present a problem at all.