Since April 2015, prospective adopters who foster their child prior to adopting them are eligible for adoption leave and pay https://www.gov.uk/adoption-pay-leave. It is likely you will be required to leave work at short notice if you have a child placed with you through Early Permanence, and therefore you will need to have supportive employers.
As a foster parent, you will be entitled to a fostering allowance from the Local Authority responsible for placing the child, which will end if the placement becomes an adoption placement.