All employees must, by law, get a �written statement of employment particulars� within two months of starting work:
http://www.direct.gov.uk/en/Employment/Employe es/EmploymentContractsAndConditions/DG_1002790 5
If you're genuinely employed on a 'casual' basis, then your employer could decide which days you're required to work. However, if you're required to turn up (in the full expectation of a day's work), then this is not 'casual work' and you should be paid for that day's work (even if the employer sends you home). That's part of the contract which exists between you and the employer. (Even though you've not received a �written statement of employment particulars�, a contract has existed between you and your employer since you were offered, and accepted, the job).
Effectively, the employer is 'laying you off'. You should still be paid:
http://www.direct.gov.uk/en/Employment/Redunda ncyAndLeavingYourJob/DG_10026693
While you have a statutory right to receive a certain number of days holiday each year, the employer is entitled to decide the days on which you can take those holidays. However, you must be given reasonable notice of those dates. The employer can't just tell you, when you turn up for work, "Sorry mate, you're on holiday today".
For further advice, contact the ACAS helpline:
08457 47 47 47 (Mon-Fri, 0800-1800. It takes ages to get through but they do know what they're talking about).
Chris