If you can persuade the accommodation provider to take a slightly reduced deposit, so that the amount comes to �100 or less, it will only cost you �12 with MoneyGram. Otherwise, the charge would be �18:
http://www.postoffice.co.uk/portal/po/content2 ?catId=19400180&mediaId=19700176
If you've got a PayPal account, you could ask if the recipient also has one. If so, just get him to invoice you through PayPal.
I send money to Russia and the Czech Republic using Ikobo. If you sent US$210 to Croatia, the recipient would receive a Visa debit card pre-loaded with HRK1135.86 (which is roughly the required amount). The fee would be US$11.30 (under �6).
Posting cash by 'International Signed For', would cost you �3.98.
Chris