To get the same speeds as you're currently getting with your BT package you'd need Sky Fibre Max, rather than Sky Fibre Unlimited (which only gives you half the speed).
However not everyone needs super-fast speeds anyway. If you've got a house full of teenagers all downloading Youtube videos, Netflix content, etc, while you watch Now TV at the same time as backing up vast amounts of data to the cloud you'll benefit from fast speeds. If there's just you browsing the internet, and perhaps watching the odd video or two, you probably wouldn't even notice if your download speed was halved.
Sky appears to be cheaper though (but I've not read through all of the small print):
https://www.productsandservices.bt.com/products/broadband-packages/
www.sky.com/shop/broadband-talk/fibre-optic/
Depending upon your needs, you might benefit from getting Sky broadband as part of their Now TV 'Combo':
http://www.nowtv.com/broadband-and-tv