First point . . .
Most workers should continue to receive full pay during 'lay off' periods, although there are some exceptions. See here:
http://www.direct.gov.uk/en/Employment/Redunda ncyAndLeavingYourJob/DG_10026693
Having got that out of the way . . .
Assuming that you were legitimately laid off without pay, and given that there's no definite date for you to return to work, your employer has probably acted wisely in providing you with a P45. It's simply a statement of the total amount of pay you've received. and tax you've paid, during the current financial year. If you find temporary work during the lay-off period, you can give it to the new employer and be taxed at the appropriate rate. Without a P45, you'd have to complete a P46, which might result in you being taxed at too high a rate.
Of course, if work fails to pick up next month (or soon afterwards), you'll have been made redundant (rather than being laid off) and you should make sure that you receive (at least) your statutory entitlements. See here for information:
http://www.direct.gov.uk/en/Employment/Redunda ncyAndLeavingYourJob/Redundancy/index.htm
Chris