Retirement allowance replaces reduced earnings allowance (REA) if, when you have reached pension age (see Chapter 46(2)), you give up regular employment and on the day before you give up that employment your award(s) of REA adds up to at least £2 a week. Retirement allowance is tax free and paid in addition to any earnings or other non-means-tested benefits you receive.

Retirement allowance is £22.15 a week or 25% of the REA you last received, whichever is lower.
SSCBA, Sch 7 , para 13