The forum staff cannot cancel your service for you, @CRGStuart20.
Take a look in Ts&Cs, linked at the foot of any TT page, and you will find detailed instructions about notice periods and the address to write to - keep a certificate of posting from the Post Office if you use this method.
Otherwise, your only option is to persevere with the phone lines.
Monday to Friday, 10am-6pm, 03451 720046
Wait for messages about Coronavirus to finish and ask to speak to LOYALTY. Say you want to cancel.
Keep your Direct Debit in place until you are sure you have settled the final bill. TT always bills a month ahead, so expect it to take a further month to reflect when you leave etc. Any overpayment can then be claimed back through My Account.