You'll find that all those folk sitting at work between 12-6pm are using a massive amount of bandwidth compared to those folk all sitting at home 8-10pm chatting online, or even downloading stuff.
Thus, the "daytime" Internet in the UK is far far slower in comparison to the evening.
You'll probably also find that your service provider gives a higher priority to their "business" traffic during the day, as opposed to consumers, who are meant to be out at work at that time
