To cancel Sky online, you can12:
Call Sky at 03337590963.
Send an email to customer support at mysky@sky.uk. Include your name, address, postal code, and telephone number in the message.
Sign in to your account to use Sky’s messaging service or fill out an online contact form explaining that you want to cancel.
Access your account at sky.com/myaccount. Login with your email and password. Once logged in, select the "message centre" option. From the drop-down box, select "your contracts." The start and termination dates of your contract are now visible.
Contact Sky at 0800 151 2747 to request the end date of your contract.
There are other options than the call centre if you were not aware - hope this helps