There is no tariff offering unlimited texts overseas. However, if the person overseas has a UK network phone, any of the unlimited text tariffs will give you free texts from the allowance. Cheapest option for you would be to send an O2 sim to the person/people that you are texting, as long as they have an unlocked phone.
I've never used that site or any other sites offering cheap sms but I'm sure someone will be along tomorrow to comment whether they have used it. Good luck with that. I would suggest a Skype app for cheaper texts but again, they aren't free.