Ultimately an enforced notice period would have to be reasonable - for someone in a upper management position for instance its quite reasonable to expect a longer period of notice - typically the full 12 weeks - for someone on minimum wage in a part time position quite hard to justify that if it came to court and the reasonable notice period would most likely be considered more in the region of 4 weeks.
I'd be inclined to agree but as most of us are in a IT desktop support type role, 4 weeks on out current contract seems reasonable.
If i speak to a recruitment agency and tell them i have 12 weeks notice to serve, they'll just laugh and hang up.