Undecided on apportionment - mobile

I have a simcard from giffgaff costing £12 per month which i then reduced to £10 per month due to lack of use…
If it wasnt for needing to be online to advertise, printing, research, sourcing material etc then I wouldnt have any…ie we dont have tv or a home phone. Hippy eh?
Im not sure if I should be putting the whole cost through or part of it.
And, as I dont make personal calls either, I couldnt use a calculation of percentage of minutes used on personal / work etc.
What is the best way to deal with it?

For the small amount i would personally use the full £10 however im sure someone will know the legal amount, but i wouldnt worry and just put through the full £10