Becky
I don't think there is an easy way to do this , via UK Bank to Greek Bank. I also think that £25 charge would stand with any "Currency House" you use.
( The same for e100 as it would be for e10,000.
You have , and again it's "I think", two options.
a. Find someone here, or coming on holiday, to give him the e100 euros on your behalf and then transfer the equivalent , in sterling to his/hers UK account.
b. Ask the owner if he really wants a deposit. - You can after all give him proof you are coming over with Flight details etc.
My last time queing in a Bank took me 49 minutes. Not something I would wish on anyone. The last time we paid a deposit for an Appt was many many years ago and he took an English Cheque.
Cheers
Negg