The allowance on commercial flights leaving from a UK airport varies in weight depending on airline and class of travel - check with the airline you plan to use. Maximum physical dimensions are similar throughout, 56x45x25cm, and you are allowed only one piece. You are presumably aware af the liquids, gels and creams restrictions. Hand baggage allowances from non-UK airports vary and are generally more generous, but not always - check with the airport and airline in question.