It's free with the packages listed on the link - if you've not got one of those, then yes, you pay for it -
"BT NetProtect Plus is free with all BT Infinity products (apart from 20GB BT Infinity 1), or if you have Unlimited Broadband extra or BT Total Broadband Option 2 or Option 3. If you've got Broadband, Unlimited Broadband, BT Total Broadband Option 1 or 20GB BT Infinity 1, you can add BT NetProtect Plus for just £3.70 a month, with the first month absolutely free."