An employer is free to determine when your holidays occur,
subject to providing you with reasonable notice.
You have not been provided with reasonable notice, so today can't count as part of your holiday entitlement. You've been
laid off. You're still entitled to receive full pay for today, with no obligation to work any extra hours for that pay:
http://www.direct.gov.uk/en/Employment/Redunda ncyAndLeavingYourJob/DG_10026693
For further advice, call the ACAS helpline. (Details are in that link. It usually takes ages to get through, but it's worth the wait).
Chris