@john6767. If you want to ignore people pointing out more considerate options of making life easy for yourself and also which won't damage someone's property, then I am sure neither anyone pointing it out nicely to you nor their "going to the authorities" will make a blind bit of difference.
IF EA states "the Round House" which you "believe", then I very much doubt they intended it as anything other than an extremely arbitrary description for the benefit of ease. The FACT is, the Round House is not on the Thames. Maybe you should contact the navigation authority to point this out! ?
So nit pick all you like but it is a boaters decision if to use the winding point, or if it is obstructed (as this one is), to use other points which won't disadvantage or inconvenience other people, of course if someone is "determined" enough then they'll try & turn wherever they please despite that. I suspect "most" boaters (95%), including I guess Mike Todd, others and definitely myself, would prefer not to upset other people or cause damage once they realise they potentially are or it's politely pointed out.... but not all. I'm just pointing out the really easy solution for anyone, which is available near the marina, one which doesn't risk you seriously running aground and/or damaging someone's property.
The Winding point "near" the Round House (and you can naturally assume where EA expect you to consider the limit for large motorised craft) is near where the River Coln meets the Thames (that doesn't mean you can't try and go further as the OP is asking) but it is not in the entrance of the Thames & Severn Canal (which is not even part of "The Thames") and not up The River Coln (you'll definitely go aground there), but make of it what you will, I doubt anything anyone else politely points out to you will make any difference