On the Impetuous British Cavalry rule at Waterloo my thoughts are
Of the two British cavalry brigades where the guards brigade and the union brigade.
The 3 regiments of the union brigade hadn't seen service in the peninsula. which explains the British Heavy cavalry unit as for the guards unit which had seen action in the peninsula maybe its to do with the fact the Uxbridge the British cavalry commander hadn't seen service since with Sir John Moore in the Courrna campaign and lead the charge instead of forming a reserve to move up in support
Also as Phill said C&C games often rates units by how they performed in that battle and during the British Cavalry charge at Waterloo they had trouble reforming.
Although this wasn't a new problem for British Cavalry but at Waterloo it went badly for them.