Cruising on the rivers in Britain is largely the same as the canals, and since they frequently interconnect, you will find that most of the boats available for hire are in fact narrowboats.
However, certain rivers such as the Thames, do have a life of their own and it is possible to hire cabin cruisers from companies based on them. For the most part, these cruisers are comfortable GRP moulded craft and identical in every way to the boats based on the Norfolk Broads.
River navigations are considered 'easier' than the canals, mainly since they have less locks. The rather more open spaces make the driving nominally easier and certainly there is less work to do, particularily as most river locks tend to be either automated or operated by a lock-keeper.
The downside to river cruising can be the delays caused by local flooding following heavy rain. This is rarely serious, but navigation can be stopped by the authorities if the river levels rise above certain safety levels.
Rivers all have their own idiosycracies and so the links below will take you to the websites of suitable operators based on each of them.