I work as a nanny for family where both parents are teachers, so I do term time only. This works out great for all of us, I get some free time to do anything I like, and they get reliable childcare during term time.
I would suggest that if you want a nanny to work term time only, you indicate salary as a yearly figure. Then a good way of splitting it up is to do a monthly salary, so the nanny gets paid regardless of if they are actually working or not. While this may seem strange, it works out well for everyone, as typically teachers are paid monthly, with salary split over 12 months, so it makes it easier to budget for paying your nanny, if you pay them on the same basis.
You will need to explain it well to prospective nannies though, as it is different to agreeing a weekly or hourly salary. The nanny is technically paid a lower rate than perhaps normal for hours actually worked, but due to being also paid during holidays, it makes it up.
Something else to suggest, as you only have one child, perhaps doing a nanny share would be possible, perhaps finding another teacher at the same school who also requires childcare on the same days/hours. This would increase care ratio to 1:2 but it does help reduce the cost, as teachers are not currently able to benefit from the cost savings of using an Approved Nanny (hope the government change that soon!).
Good luck in finding a nanny.