Ooh, this is useful:
Do I simply raise January invoices at 17.5%?
The short answer to this question is almost invariably “yes”. But if you have a lot of invoicing from early December to catch up on, then supplies made before 17 December will be subject to the old rate of VAT. This is because for most types of supplies the “basic tax point” is the date on which the supply was actually made, and if an invoice is raised more than 14 days after the basic taxpoint then the taxpoint reverts to the original date of supply rather than taking the invoice date. So if you are up to date with your invoicing by the time you take your Christmas break, then no problem at all.
You can read up on this here.