A ship of the line is a Double or triple decked battle ship from the 17th through the mid-19th century like those used at the battle of Trafalgar.
Ideally I'd like to crew on one for a week or so, failing that just an opportunity to sail on one.
HMS Victory at Portsmouth, while technically still part of Her Majesty's navy, is no longer afloat so is out of the question.
"tall ships" is the magic google-incantation.
The ships are probably more modern than you want.
It appears from @KateGregory's research there are no longer any original ships of the line sailing in Britain or the world.
I have found there is a 6th rate ship of the line(frigate) replica named the Grand Turk which is still sailing, used in the ITV series of Hornblower.
Also of interest for Australians and New Zealanders, there is a replica of HMS Bark the Endeavour
Sadly there are no original ships of the line still sailing. Probably the closest is the Grand Turk.
Checking for ships used in movies is a fruitful source of replica fighting sailing ships. As an example HMS Surprise is a replica of a sixth-rate post ship HMS Rose - not a ship of the line, ut at least a genuine fighting ship.
In Sydney at present, they're advertising the historic Tall Ship - James Craig, as sailable for a day. It's technically a barque, but is the original fully restored real deal - one of only four still sailing in the world - originally called the Clan Macleod.
