1. As per the form only enter a Home office reference if you have one
Normally these can be found in cases where you may have received official immigration letters from the Home Office.
2. The payments section only requests "Cardholder's signature" for payments made by either credit card or debit card.
Means the signature in that box should be exactly the same name and signature as that on the card being used for payment.
This may or may not be the applicant.