The last time we hired was about 10 years ago, so I'm not in a position to recommend particular companies, but I can add a bit on the weather. We hired for the last week in October, 3 years running: it's a good time in so far as the canals aren't so crowded then (in high summer some locks and junctions can get
very slow). Do bear in mind, though, that a narrowboat's steering posision is out in the open. With warm clothes, a waterproof hat/hood, and
warm gloves, we were fine: regular cups of coffee from the galley helped, too!
It's worth hiring a boat with a "traditional stern", rather than a "cruiser stern": the former offers the steersperson far more weather protection. And if you can find a boat with a wood/coal burning stove in the stern cabin (as was the case in the old working boats), the warmth coming up through the stern doors makes a real difference. 10 years ago, Brummagem boats, based in Birmingham, used to do these, but a quick Google suggests that they're no longer in the hire business.
This site shows pictures of the stern designs that I've been banging on about. It actually makes a BIG difference to winter cruising, we found - but "real" trad (as opposed to the far less comfortable "semi-trad") are fairly few and far between in the hire fleets, so be prepared to search!
If there are only two of you, the non-steerer (when not making cups of coffee) can sit on the rear cabin roof, with legs dangling down next to the warmth from the stove. Very cosy for both of you.
Beebee was quite right to warn about the weather - but, given a careful choice of boat, I think you can have a great time.
Jonathan