Like SwanR, our experiences of motor cruiser hires were in March, May and October. We like that it’s quieter, but generally found the weather better in the Autumn, the nights during early Spring are generally colder than those early Autumn. We have now bought into a syndicate boat and enjoy all seasons on the river (except when it’s very windy...and that can happen any season).
Also, if you like less busy rivers, consider hiring from a yard on the Southern rivers.