Hi Scott, a compromise between a Narrowboat and a river boat would be a narrow beam GRP boat such as an Atlanta 32 or Viking 32, both of which, usually, have an outboard (petrol), would allow you access through 7ft wide locks, and would have reasonable 'sea keeping' abilities on the tidal bits of the Thames.
The mooring fees on a narrowboat could well be more than a GRP boat too.
If it isn't going to be a liveaboard, just a weekend and holiday boat, you could have a lot of money tied up in a narrowboat when it's not being used, if you see what I mean!
I have a widebeam Viking 24 with a 30hp outboard, and the fuel consumption at river/canal speeds is around 1-2 litres an hour, pretty frugal really, although when opened up (pointless as the top speed downstream with a 30hp is about 9mph!) the consumption rises proportionally.
It's all about what YOU want, and what you'd be happy in.