Without running to look up my notes etc I can offer the following.

Throughout the campaign, Wellington left the details of handling cavalry to Uxbridge.
They had never served before, due, most say, to the scandal with Wellington's niece.
Nevertheless Wellington did not really show any marked displeasure at his presence, he often made him the butt of his rather acid sense of humour, he would have preferred Stapleton Cotton.
This is evidence not of personal dislike but of a preference to work with officer's he knew. The Duke complained at the start of the campaign that he knew very few of his officers. In the age before telecommunications and radio, knowing how a man would act was a vital part of command.
Wellington commanded a large force of heavy cavalry in 1812, when the heavy brigade under General le Marchant crushed a French division, being sent forward by Wellington's personal command.
Wellington disposed his cavalry at Mont St Jean so they would be in a position to support the Infantry closely. There may have been an intimation to Uxbridge at some point that he was to charge whenever he saw an opportunity.
On the day of the Battle, accounts from both the allied and French sides suggest strongly that Picton's division was wavering, and indeed battalion's of Pack's brigadein the centre were stepping backwards down the slope from the hedge.
On the right flank, Picton had lead forward Kempt's brigade, momentarily checking the French but then too wavering under the heavy pressure. At this point we can see that things were becoming dicey because Picton was now dead, and guns were being abandoned as the French advanced.
While all of this had been going on Uxbridge had been watching the French come on. Wellington by my reconing was not on the spot. Indeed both the Duke and the Earl were initially over on the right flank, and Uxbridge was first to arrive on the scene.
He at once rode to each brigade of the union and household brigades and ordered them to be ready to advance. The cavalry were in motion at the perfect time. Uxbridge gave the signal to charge as the Infantry began to fall back and the heads of the French columns began to crest the bank of the road. At this instant, Uxbridge directed his staff to order the attack, and personally lead the household brigade to the charge.
Uxbridge forever blamed himself for not riding with a formed reserve to avert the destruction of the union brigade that followed.

In sum, Wellington did not order the cavalry forwards, but had anticipated the need for them to do so. Picton's division doesn't seem to have been able to have held on much longer without some sort of intervention. Uxbridge was the man of the hour, but also thus bears responsibility for the heavy casualties incurred upon the heavy cavalry.

Josh.