Not the whole London to Nottingham, just the last maybe 20-30 minutes from where the line was blocked overnight for works. And they obviously do often charter buses, in fact my local train operator was a bus company as well so their buses got used for this type of event because it's just internal accounting. However in the example I gave above that operating company had chosen not to hire a larger vehicle because they anticipated low volume. Six taxis is probably cheaper than a coach. A hundred not so much.

They had bad luck, a different train hit a person (almost certainly a suicide, it is possible to get struck by accident but it's not common) and delayed a large amount of passengers like me who were going to London to get that Nottingham train, people delayed by that incident from their last-but-one train [which ran normally all the way to Nottingham] filled this slow, train that couldn't get all the way instead. A really smart organized team in St Pancras could have realised way too many people are boarding that last train and warned their colleagues, but realistically it was probably already too late to organise a better response even if somehow an incredibly joined-up organisation had reacted to the problem.