Go for Mandatory Reconsideration(it’s really rejection but they can’t call it that) as this is the 1st step and you HAVE to go this route in order to be able to appeal, once you get the rejection letter make an appointment with the Citizen’s Advice Bureau
https://www.citizensadvice.org.uk/about-us/contact-us/contact-us/contact-us/
They will help you with the appeal process.