Phileas,
Sorry, this is incorrect. Below from the DfT website:
Cabin Baggage
ONE item only of cabin baggage is permitted through the airport security search point, the dimensions of this item must not exceed: a maximum length of 56 cm, width of 45 cm and depth of 25 cm (including wheels, handles, side pockets etc.). Other bags, such as handbags, may be carried within the single item of cabin baggage. All items carried by passengers will be x-ray screened.
The link to the page is here
http://www.dft.gov.uk/stellent/group...ut_612280.hcsp