if you can buy the boat, you can pay the tax... lol, but as an aside you may be able to flag her offshore. The only reason I can imagine that Florida capped boat tax at 18k is that they were losing too much revenue to this:
Florida boat sales tax cap goes into effect on July 1, 2010 - Casualties - SuperyachtTimes.com
but IIRC if moored in the states, flagged offshore, you only have to make one trip per annum outside international waters per year, snap a photo of the gps (some proof of vessel location).
boatloanworld.com - OFFSHORE REGISTRATION - Recreational lending for boats & (RVíS)
I do not know how EU/UK regs apply, am not a tax attorney and if you look into this you should make inquires with your lawyer (barrister?), I am qualified to state my thoughts, but not to cite the technical legality of the issue.. standard disclaimer... yadda yadda