Gidday Mate,
Spent late 2010 to late 2011 researching the Kennet & Avon max allowable size.
After contacting Richard at Waterway World Mag, the Lady who runs the K&A Trust, people that have a Restaurant boat on the K&A and reading 20 different web sites we were able to freeze on 70' x 13'6" x 7'8"AD as definitely able to pass without trouble. As our water draught will be only 18" depth was immaterial.
(No ballast - why spend money to build unneeded hull and yet more money to push an unwanted 5 tons around?)
As for the canals we don't fit thru - we'll go around - Category C or B.
Also there are several canals at 13'6" so it seemed a good cut-off point.
Cheers! Rewi