it seems there must have been some confusion here when you previously spoke to VM.
As part of the agreement you take, your monthly payment will continue until you request to change as per the agreement:
3.4 At the end of the Minimum Term - "You will need to contact the Team at least one month before the expiry of your Minimum Term or any subsequent monthly renewal date for your Contract Allowance if you do not want us to continue provisioning you with your Contract Allowance each month. If you do contact us we will not provision you with your Contract Allowance but will continue to provide you with the Services at the rates set out for Pay Monthly customers in our Tariff Table until this Agreement is cancelled by either you or us in accordance with Clause 10."
You need to contact Virgin mobile (I believe the number from abroad is +447953 967 967) & clearly cancel or downgrade your package.