To register with OFSTED they will need to:
do a common core course like the ICP or MNT's distance learning version - cost around £150
do a paediatric first aid course - cost around £100
get insurance - cost around £70 (and they need to pay for that themselves)
have a CRB etc done which means they need proof of residence at your address, forms of ID etc and which is really rather pointless assuming this is the first time in the UK as foreign police histories aren't checked unless it's some international crime
pay £100 odd to OFSTED to actually get regustered
[pray that you've done everything correctly, sent off all the right forms, they haven't changed the rules....]
So count 2 months to meet all those requirements, then 3 months to get registered plus all the cash you have to shell out... may not be worth it.
As Treeesa says, level of English will also be a consideration as they may not pass the common core or first aid if their English isn't up to scratch. So my general verdict would be don't bank on using childcare vouchers for your au pairs.