Concentrating one's army is not bold. However, there were great costs inherent in keeping an army concentrated, and thus units were bivouacked and billeted across wide areas. One would not concentrate an army until one had intentions for its use.
On June 12th, Gneisenau wrote that the likelihood of an attack had almost disappeared.
On June 13th, Wellington wrote that they (the allies) were too strong to be attacked.
Had the dispositions of the Allies not changed - had the invasion commenced with the Allied armies positioned at 3 am on June 15th as they were at 10 pm on June 14th, then the French would have easily captured the Nivelles-Namur road, including Quatre-Bras and the Sombreffe position. With no sizable Prussian presence west of Namur, Napoleon could (and based on his correspondence would) have thrown the majority of his strength up the Brussels road. What would Wellington had done, especially with little cavalry?
Had the campaign not been delayed, then the above would have been true, only earlier.
Before you debate the above, I challenge you to simulate the campaign using your method of choice...
What changed between June 14th at 10pm and the following morning? That will be revealed soon...
It was nothing Wellington did - and thus we can accurately say Wellington was utterly surprised.
The Allies won! Napoleon was defeated! And hence recent revisionists have even claimed the Prussians were not defeated at Ligny, but only retreated to suck the French army further into Belgium where the Waterloo battlefield had already been selected! EVERYTHING WAS A MASTER PLAN - proof? They won, didn't they?
From Albert Sidney Britt, an alternative point of view:
If there was lack of urgency in Blücher's army, there was downright indolence in Wellington’s headquarters. No small part of it was due to the British commander himself. Having made the assumption that Napoleon would not attack, he spent most of June 14 and 15 squiring a succession of women to various social events in Brussels. When a message arrived on the evening of the fifteenth reporting Blücher's concentration around Ligny. Wellington merely ordered a southeastward shift of his forces located west of Brussels, but gave no indication of the purpose of this movement. Having dashed off these orders, he then left for the ball being given by the Duchess of Richmond, where he remained until 2:00 a.m. on the following morning.