The £85,000 limit on compensation is still covered by the Financial Services Compensation Scheme (which is in fact covered by a levy on other financial institutions not the Government should a bail out be necessary).
Moody's downgrading, together with any similar downgrading by other rating agencies, such as Standard and Poors, only really affects the interest rate that the downgraded financial institutions have to pay when they borrow money from other banks, as their credit rating is deemed to be lower.