From the DFT:

Each passenger is permitted to carry ONE item of cabin baggage through the airport security search point. The dimensions of this item must not exceed: a maximum length of 45 cm, width of 35 cm and depth of 16 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.

well - it's an improvement...we have 'lost' a number of laptops in the last 3 days from checked luggage. Thank goodness for encrypted filesystems!