Your 'grandfather rights' c1e entitlement will be limited to 8250kg total, which is unlikely to be enough or a 5 ton boat and whatever vehicle and trailer would carry it.
Even the best tow cars top out at 3500kg, and 7.5T wagons usually only have a cargo capacity of about 2.5-3T max. Not really anything that takes that on your license. Take a long holiday and boat it down?
I once towed a 27ft Bayliner on a trailer behind a 7.5t empty recovery truck . That was a long rig ! I only had to move it from a workshop to a local marina a distance of about 8 miles . It is not an experience that I will ever willingly repeat .