You could contact Guys & St Thomas Charitable Trust to make sure they're not legitimate.
Joint Director of Fundraising for Guy's and St Thomas' Hospital
NHS Trust and Guy's and St Thomas' Charitable Foundation
First Floor, West Wing, Counting House,
, St Thomas' Street
London SE1 9RT
(that's the longest address i've ever seen)
Tel: 020 7188 7711
Then I suppose if you're sure they're fake then it's a police matter.