Yes, you need a letter from your in-laws signed by them. I did that when I lodged my ILR application. I used my Mom-in-Law's checking account to pay my application as I did not have any debit card that time.
I don't think CFO will be a problem if you married here in the UK.
Much better if you contact your nearest Philippine Embassy consul.
http://philembassy-uk.org/contact.html
Send them email and tell them honestly about your CFO. They usually helps filipino overseas so not to worry.