I'm a school bursar and we've operated vouchers for ages, but have just had to stop operating them for teachers because the DfES, on behalf of the teachers pension scheme has ruled that such "salary sacrifice" schemes are invalid and the amount sacrificed can't count as pension.
This is really stupid - and it's a big embarrasment for the governmant that pension schemes are putting this obstacle in the way of staff getting their tax and NI breaks on £50 per week of childcare. I believe this also is the case for NHS pensions, too.
Many councils (who are the teachers' employers) are trying to set up schemes, but have stalled because of this problem. They could set them up, but teachers wouild have to be excluded. They are trying to resolve it ...the feeling at my local council from the chap progressing the scheme is that it will be resolved eventually.
If you are a teacher and want these tax breaks, then kick up a fuss with your school/LEA...to put pressure on the government. But beware if you had had vouchers before now (when you saved NI on the whole voucher amount) - the advice at teachers pensions is to backtrack over your salary history and refund you some pension contributions, thus possibly affecting your salary!
To non teachers - hassle your employer to get the vouchers through which you can claim tax breaks - we use Busy Bees (search on google). They are easy to administer, but I have to say the company/school doesn't now save much money - the admin costs wipe out the small savings.