Less than £20! Ouch. I'd stick with Medium - at least you can write whatever the hell you want.

There is another way of looking at it, I suppose. I did some free work for a local magazine just to get some work in print. It then gave me a few things to put on my website when I started working seriously as a freelance writer. You could just see it as a way of getting your work out there, but I'd still end that contract ASAP.