Hi ​@Loretta Cutajar, I am sorry for that. You do not need to cancel your pay as you go sim. To cancel an ID Mobile pay-as-you-go SIM, you don't need to take any action, as the SIM will expire after six months of inactivity. If you want to switch to a new network and keep your number, text STAC and your date of birth (DDMMYY) to 65075 to get a STAC code, as detailed on the iD Mobile Community and theroams.co.uk. You can also do this through the iD Mobile app or website, or by contacting iD Mobile's live chat support. 
If you want to keep your number
Text for a STAC code: Text STAC and your date of birth (in DDMMYY format) to 65075. 
Use the app or website: Log in to the iD Mobile app or My Account online, go to the 'My account' page, and click 'End my plan'. 
Contact live chat: Use the iD Mobile Live Chat to request your STAC code. 
If you do not want to keep your number
Do nothing: Your pay-as-you-go SIM will be automatically deactivated if it remains inactive for six months, according to this Three support page. 
-Isa🎀