I assume you got flashed by a speed camera? Officially on a UK licence up to 20mph over the limit means 3 demerit points and a £60 fine. More than 20mph over the limit means 6 demerit points and at least a £60 fine, I don't know what the standard fine is at that point though. More than double the speed limit or over 100mph generally means a court summons and an unlimited fine.
To be honest, if you were flashed by a Gatso camera there's only a 1 in 8 chance (official figures) that you'll get the brown envelope. Often there's no film in the box and other times Officer Dibble just can't be arsed with the paperwork involved. If you get it, it'll arrive within 2 weeks of the incident.
You might get lucky having an international licence and just get the standard fine, or perhaps nothing at all if it means more paperwork for the copper. A friend has a Swedish licence and a UK licence and gets stopped for speeding almost on a monthly basis. Every time she's stopped in either country she shows the opposite country's licence, feigns not being able to speak much of their language and they just wave her on!