The RNLI and MCA can no longer accept out-of-date flares from the general public, Expired marine pyrotechnics are classified as explosives under the UK’s Explosives Regulations 2014 and so they must be disposed of responsibly.
If you have flares which you need to dispose of you can find more information on the following government website - https://www.gov.uk/guidance/disposing-of-unwanted-marine-flares.