The computer will 'count down' to your next service regardless of whether you are on fixed or variable. it just serves as a reminder for fixed interval cars.
If you have the original drivers information pack then at the front should be a sticker with a grid of numbers and letters. This provides an 'at-a-glance' reference for any dealers to tell them what equipment you have installed on your car. Under the servicing section, it tells you how to identify if you are on variable service by looking at this grid for a particular set of figures, if you have them you are, if you haven't then you're not!