DBS with Update Service is one of the things you need for Ofsted registration and is done via the Ofsted DBS application system. You will also need the First Aid course (I expect you are doing something suitable, but maybe check with the provider) and some kind of Common Core course, or something Ofsted accept training wise. Plus then Nanny Insurance.
Look into what demand there is locally for nannies. Sites like Childcare.co.uk and Nannyjob.co.uk list jobs and can be searched by location/area. Get a feel for the typical ages of the children which parents have who want a nanny for the hours/days you are able to do. I expect it will be parents with babies and toddlers, perhaps with an older child at primary school. Once you have a feel for the local market, adapt your CV to fit (and then customise for each job you apply) so that it clearly shows your experience with the age groups concerned.
Your current foster children sound as though they are quite old and they may be long term fostered by you. So look back to those times when you fostered a baby/toddler. Look back at previous work with babies/toddlers such as whatever you did before fostering. Even look back to your teenage years, did you babysit? Did you help at Brownies/Cubs/any kind of youth group? Did you help at pre-school/nursery, or work at one? The more things you can show that you have done in the past, the more it will help parents feel that you are able to care for their children. The fostering itself is a great experience to have as it's 24/7 and may have been many placements with a variety of ages of children.
Having your husband around to deal with anything your foster children need is useful as that will mean your hours are not so tied to being only within school time. Consider what would happen during school holidays.
Look at other threads on this board about employment rights and things to do with contracts. There are many many pages in this section of Mumsnet, so scroll down and have a look for anything about nannies. Get a feel for what you would be expected to do as a nanny.
It's quiet on Mumsnet this time of year, so stick around and ask questions plus contribute to threads as I am sure your fostering knowledge will be useful to some of us, or just be interesting - what made you decide to foster children?