This might not apply to everyone, but when I logged into my Sky account last week and viewed the 'Offers & Upgrades' it was offering the 'Ultimate on Demand' for £6, with UHD for £1 on top. (Ultimate on Demand is Sky On Demand & Netflix bundled together).
Nothing completely amazing there, but when I selected the Ultimate on Demand, it only showed my monthly price increasing by £3 not £6 and greyed out the £1 UHD add-on and said 'You already have this'
Once I proceeded through and viewed all the billing, whilst it did add £6 for Ultimate on Demand, it dropped my Multiroom sub by £1.50 and converted my Sky Entertainment to Sky Signature and made that £1.50 cheaper as well.
So in short, I got Netflix Premium for £3 a month and just linked it to the Netflix Premium Account I already was paying £6 a month for.