A quick search on t'internet reveals the answer as given above by nicknorman. -
Speed limit varies depending on the river in question and depending on the expected flow. Speed is measured 'over the river bed'.
Low flow sections have a low speed limit, often 4 or 5mph, and generally the same in either direction. High flow sections have higher limits, typically higher downstream than upstream. For example, the Avon; 6mph downstream and 4mph upstream. This caters for the higher speed over the river bed (being an aviator, I'd call it groundspeed), when motoring downstream. Tidal sections on some rivers are significantly higher limits, e.g. 15mph on the Great Ouse.
I guess the significant point here is that you should check before you go. EA website has information on the rivers under their juristriction, and others are only a Google search away. Be aware of strong stream advice too. I learned a sobering lesson about that last year on the Stour!