In reply to Green Porridge:
+1 for the Four Counties Ring. It can be hard work, but with 4 people and long summer days it should be manageable. (We did it as a 2 after the clocks changed in October.) Get a system going, with one person setting locks on a flight (if needs be) and it'll be okay.
Otherwise the Leeds-Liverpool canal, which is really good too. If you'd been before, I'd suggest the Huddersfield Narrow, but I wouldn't recommend it for a first trip. You can do a one way on the Leeds-Liverpool as hire firms will arrange minibus drop off/ collection.
Obviously avoid the peak holiday season if you can but remember that boats tend to be 'tired' towards the end of the season and often there are more would-be hirers than boats on the more popular canals and boats can sell out quickly. Hire costs have gone up a lot in recent years, far outstripping inflation, and you don't necessarily get what you pay for. There are big, multi-base companies such as Anglo-Welsh as well as smaller independents. Pros and cons with both. Be wary of hidden extra costs - e.g an en-route pump out.
Hoseasons.co.uk might be a good starting point.
You can generally moor up anywhere unless there are signs saying not to. No boating after dusk or pre-dawn, but you'll usually see this rule broken by folk who've mis-timed a leg and need to reach a particular mooring. Use common sense when drinking alcohol, especially when working locks.
Post edited at 12:39