We went on a narrowboat on the canals in Rugby, near Coventry last year.
8 of us, mixed group ... think we were 18 - 20ish...
Was a cracking hol. We only went for 3 nights, Friday - Monday but it was fun. Just got wasted every night doing renditions of Lional Richie and dancing on the roof of the boat with the music blaring

You can moore up wherever you feel is reasonable so you can get quite some distance out.
Our boat was massive, one of the biggest in the yards fleet. Never driven one before but picked it up really quickly, it's pretty easy after a few hours of getting used to opposite steering and using the reverse gear effectively to drift into corners.
Few things with it... i'd defiently advice planning your route and driving times. I know people like to do things spontaneously.. and i do too, but you've got to consider wherever you go you have to turn around and be back, so effectively on a 3 day break you've only got 1.5 days of going in a particular direction. Best to set an aim (deffo buy a book on the local canal) and plan some stops.
We were quite dissapointed by the lack of decent canal pubs on our route, it was a bit sparse and there weren't many. We did find a few and made up for it by going into Coventry for a night out, etc. Make sure you stock up on beer for the whole trip as there's nothing worse than realising you are running out and not having a clue where the nearest supermarket is.
Another rule is you can't use the boat after dusk or before dawn, you need to be moored up. Although we did a fair bit of midnight driving

The person who's in charge of the boat should be sober as well.. but this never got checked for us, and I was driving along with a beer in my hand.
We went with Hoseasons and I forget the name of the boating yard.. cruisers or something. You can park your car in the yard and it's secured, etc.
Really good, deffo reccomend!!