Hey there,
My husband and I went narrowboating 1.5 years ago. It was fun, but here are my observations: the double bed was very narrow - you're better off getting single beds for everyone. Also, don't rent the cheapest boat you can find! We did, and the motor was VERY loud.
If you check the websites that aggregate the available boats from various companies, you can see that some of them have ratings, and all of them will tell you which amenities are available onboard! Finally, buy a guide to whichever canals/rivers you'll be on. We borrowed one, it was invaluable for telling us our next source of water!
The only narrowboat rental in Oxford proper is College Cruisers, located in the Jericho neighbourhood, at the start of the Oxford Canal. (We didn't rent through them.) There must be lots in Banbury, however...?