I'm in London zone 2 and pay £11.50/hr gross (roughly equiv to £9/hr net). However most very experienced nannies seem to want £10/hr net which is more like £13/hr gross.
Rate depends on experience but I think it also depends on how attractive a job you are offering - eg full time is generally attractive, after school positions not so attractive as it's hard for the nanny to get up to full time hours that way iyswim. Your two days will be more attractive if you can be flexible about which days, to fit with a nanny who already has a three day position.
Avoid using an agency if you possibly can as it will cost you £1000s extra. We used a local online forum site.
Don't forget hidden costs which don't apply to nurseries - eg heating your home, nanny's food, paying for nanny to do activities with child, etc. on the other hand nannies will look after sick children unlike nurseries!
Nannyshares are very very common and could be a v good option for you. going rate seems to be that each family pays about £6 net (not sure what that is gross). Obviously you would need to find a compatible family and make various compromises though.
Childminder could be an option? Bit more flexible than nursery I believe (ie you might find one willing to do longer hours) for a similar cost.