Yes, length, that is for a 12 metre boat. Many costs on a narrowboat are determined by length, licence, cost of blacking, insurance etc. The Medway is more geared towards grp cruisers than narrowboats, grp boats tend to be much shorter.
I personally wouldn't travel to or from the main network from the Medway on my own, or on anything other than the calmest day. narrowboats are designed to float in muddy ditches four feet deep, not tidal estuaries. I have been up the Thames numerous times on the Cobelfret ferry and even that has difficulties on occasion.