Yes! You do need a valid UK (or international) motorcycle licence to ride one of these tours. You can only ride an off-road motorcycle on private land without a valid motorcycle licence, so a licence is definitely needed in Spain.
You will be required to bring your licence with you and carry it at all times when you are out riding the motorbike.
Please advise us of the type of UK licence you have to ensure that your licence covers you for the correct size engine size.