Ofcom regulates the advertising on UK commercial TV stations thus:
ITV, Ch4 (S4C) & Five - The total amount of advertising in any one day must not exceed an average of seven minutes per hour of broadcasting. Because it is an average figure there is certain freedom to have increased breaks throughout the day balanced by shorter breaks at other times. However, between 6pm-11pm (and 7am-9am) the maximum break is limited to 8 minutes per hour.
Other channels under Ofcom control - The total amount of spot advertising in any one day must not exceed an average of nine minutes per hour (15 per cent) of broadcasting. This may be increased by a further three minutes per hour (5 per cent) devoted to teleshopping spots, but this additional 5 per cent must not be used for other forms of spot advertising.
http://www.wwww.radioauthority.org.uk/tv/ifi/c odes/advertising/rules/rules.pdf
It makes sense for channels to increase the length of breaks at peak times i.e. when most viewers are watching.
Ofcom are currently considering a change in the rules.
http://www.telegraph.co.uk/news/uknews/1582288 /Ofcom-considers-increase-in-TV-adverts.html