I'd be inclined to split the package.
Assuming the Johnson is a 2-stroke V4, then you'd probably see up to £1,200, depending on condition. The recent work all helps. Mid-sized 2-strokes up to 90hp command a better premium, especially in Yamaha guise.
Bill Higham is expensive for his used outboards, but gives you some indication of prices. Used Outboard Motors for Sale | Used |Second Hand | Outboards | UK
Seamark Nunn have a good selection too: http://www.seamarknunn.co.uk/download/usedob/used.pdf
The boat and (assuming there's a trailer too) can be sold as project. Price-wise will depend just how much of a project we're talking about. A re-tube for a 6.2m boat will probably be in the region of £6k+. If the tubes still hold air, but could benefit from new wear patches and a general tidy, then there's money in that too. If the deck is goosed and soggy, then you're looking under £500.
This is quite a good site to sell the RIB: Ribs inflatables for sale - FAFB
or indeed Ebay.