If childcare in your home were a spectrum then your typical au pair would be at one end and a nanny at the other.
Au pairs are generally young, inexperienced, unqualified there primarily to learn english and work as a means to that end. They typically do 5 hours a day for £70 a week (living in) and generally can have sole charge of school aged children before/after school or very short periods with a baby but shouldn't be used for extended periods.
Then in the middle you have more experienced/potentially qualified in their own country foreign childcare's who want to work as an au pair to learn English, or EMT au pairs who want to gain childcare experience. They cost slightly more, typically just under the tax threshold, and might be okay with extended sole charge/care of a baby.
Then you have unqualified but experienced chikdcarers (including some ex au pairs who want to go into nannying) and newly qualified childcarers in the price band above who will work full days, can live 'in or out but cost more for the longer hours/more responsibility and additional skills.
Finally you have very experienced, probably qualified, career nannies who typically live out and earn top whack.
If the nanny is quoting £11net/hour then that's the top end of the spectrum and probably completely capable of sole charge of a baby. An au pair I wouldn't personally use for sole charge of a baby unless they were qualified in their own country and had some experience of English childcaring styles. There are significant cultural differences - for example formula preparation - that I wouldn't be happy risking and a typical au pair would know what to do with a baby all day very day. Au pairs come here for the language and the experience, they want to go to classes and make friends. Those who work a lot, unless they're childcarers already, often become unhappy.
Any live out childcarer needs to be paid at least min wage so a live out au pair type person would be more, and you could get a better nanny for not much more (£11 being v expensive!).
Don't feel pushed into having an au pair. What do you need childcare for?