According to Ikea Manchester store Sale up to 50% off starts on 26th December 2010! Also VAT increase will not apply to Sale prices “IKEA UK will increase the cost of all products in line with the government increase Value Added Tax (VAT) with effect from Tuesday 4th January 2011.
Sale prices are not subject to an increase from VAT changes. Sale prices are valid until Sunday 16th January 2011.”

http://www.ikea.com/gb/en/store/ashton/services

* It’s possible that different stores start at different dates, but it’s usually nationwide.