I think it's better to avoid using the products or services of any company whose name appears on Manchester United shirts. I have always tried to boycott United sponsors and I'm sure many other people around the UK must do the same, because although they have a large fanbase, they are probably also the most unpopular club in the country. I sometimes wonder if companies who want their names to be associated with Man U are really aware how disliked they are.
That said, it is a bit daft to cut off your nose to spite your face, if you really need to use a product or service that is associated with the Scum. Aon is one of those companies that are so large that it is difficult to avoid them! For example, they pay my pension on behalf of my former employers. I am not inclined to refuse to accept these payments on moral grounds.
Aon are the second-largest insurance brokers in the world and it must be the case that they offered the club the best deal. I wouldn't criticise the club for dealing with them if it saved them money.