Do you have Power of Attorney for your Father in Law?
If you do not have that it will be difficult for you to cancel or deal with anything with regards to your Father in Laws BT Account.
What Power of Attorney (PoA) does BT accept? | BT Help
once you have that registered with BT you will be able to act on his behalf.
If you Father in Law is still in a Fixed Term Contract I believe what you have been told about cancelling is correct.
You may be able to change to a different package for the TV and move onto a Pay as You Go call package for the landline phone which may help with the costs.
How to change or cancel a BT phone, TV or broadband package | BT Help
When the Fixed Term Contract is finished in April it will all roll onto a thirty day contract where the account will lose any discounts that it may have so you may wish to terminate everything at that time.
As long as you give the thirty days notice there will be no fee to cancel. You can start the notice 30 days prior to the Fixed Term cancellation date.
