If you have a bank account you could use a bankers' draft, although there is a fee for that service, then send it with a letter to
Marie Curie
East and Mid Lothian and Borders
Great Michael House
14 Links Place
EDINBURGH
EH6 7EB
If you know their Bank Account Number and Branch Clearing Code, you can pay in the cash at your local bank branch. Being for a charity, hopefully they wont charge a handling fee.