Actually all TV broadcasters are limited in the amount of adverts. The maximum is 12 minutes per hour. There is also a daily limit of 168 minutes (average 7 min/hr) for ITV franchises, Channel 4 and Channel 5, or 216 minutes (average 9 min/hr) for everyone else. It goes without saying that trailers and other continuity do not count towards these limits.
ITV in particular have for the last few years been selling no advertising at all between 00.30 and 06.00, so they can make their primetime adverts longer (usually right up to the limit of 12 min/hr) without going over the daily cap.
Regarding American television, it seems there is no general limit, although programmes aimed at children have a limit of 12 min/hr on weekdays and 10½ min/hr at the weekend.