Is there no internet café/business centre in targovishte? of not, try going to your lawyer and asking of you can please use the internet for this one thing.
Otherwise, transfer funds for the flight to someone you know in UK and ask them to book for you
booking from Bulgaria you will need a credit and not debit card - unless anyone else knows differently - if they do I am sure they'll not be backward in coming forward to correct this info!
Good luck