There are two different considerations.
Firstly, security rules:
When the rules about liquid in hand baggage were first introduced, everyone was limited to just one item of hand luggage. The restriction has now been removed at most UK airports, but not all. For example, the 'one bag' rule still applies at Luton. For a list of UK airports where the restriction has been lifted, see here:
http://www.dft.gov.uk/transportforyou/airtrave l/airportsecurity/cabinbaggage
Secondly, airline rules:
Individual airlines are free to make their own rules. In general, the budget airlines (e.g. Ryanair) all restrict you to just one item of hand baggage. (Ryanair are particularly hot on enforcing this rule). Some 'full fare' carriers allow two items but, with several operators, this might only apply to passengers holding certain types of tickets (e.g. Business Class).
The only way to be sure is to check with the relevant airline's website. (Even if the airline is happy to allow two items, and you're flying from a UK airport where this is permitted, the overseas airport might still have a 'one bag' rule for the return flight. The airline's website should have full information).
Chris