There's been a couple of occasions where I've been running short on strips. If it's been a weekend when GP is closed, if I go to a pharmacy which I've used before and they have my details on record, they've got a way of just issuing an emergency batch and then they sort it out with the GP later, at no cost to me. However, I tried it once during the week and they said, no go, if the GP is still open, would need to go there (so I just left it till after 6pm and went to another chemist and got them).
I suppose it'll depend a lot on how accomodating the particular chemist is but if they've got you and your GP details on record, it does seem that there's ways of getting an emergency supply at no cost from the chemist.