If you want to pay a Nanny £100 per day net then this will be £100 5 52 = £26,000 net.
Using this calculator to check you'd need to pay the nanny £35,000 gross, Nanny would pay tax of £5,705 and NI of £3,220 which would leave them with £26,000 net.
On top of the £35,000 you would pay employers NI which would be £3,748 = total wages bill of £38,748.
Using the same calculator to check if you earned £55,000 you'd pay tax of £11,930 and NI of £6,308 to leave you with take home of £38,760 to pay the nanny!
You'd need to do the payroll yourself or pay a payroll agency and you'd have to pay out extra to cover sickness and holidays.
Perhaps if you post on the Childminders/Nannies topic you can confirm whether the costs you gave for a Nanny salary are accurate.
As to how childcare vouchers work - first your Nanny would have to be Ofstead registered - I understand that most aren't so you'd have to help him/her to apply.
Then both yourself and your DH could request that your employer pays part of your salary as childcare vouchers. The maximum amount each parent can claim is £243/month so £2,916 per year. Assuming you and your DH are 40% tax payers you'd be saving tax at 40% and NI at 1% so you'd be better off by 2£2,916 41% = £2,391. (The vouchers are disregarded for calculating your tax and NI - so you can playabout with the tax calculator).
I also work freelance and can sympathise with the issues you mention.
Maybe come over to the Freelancer's topic and see if anyone can inspire you to increase your motivation/income.