Agreed, cancellation is the way to go. Within 30 minutes of cancelling I had a 60% discount offered through My Account on the Sky box itself. Online was only offering 50% so make sure you access your account via the box itself. I negotiated Sky Q separately and thought £100 was reasonable for the install but obviously there is better to be had.....like cj's zero. I think there are plenty of good deals available for existing customers, you just have to go through the cancellation palaver to get them. As I read on another forum, the mantra when dealing with Sky is that polite persistence pays.
Sky Q has been flawless for me contrary to a lot of reporting and to my eyes the upscaling of the HD transmissions is superb. There's not much UHD content as yet but that'll come gradually.