Hi, I am completely new to computer cartography, but I want to progress from pen and ink!

I currently base my hand-drawn fictional maps on UK Ordnance Survey maps. Assuming I can afford to buy the OS's digital maps, can anyone suggest the best software for me to shift to digital mapping? Or would it require really expensive commercial mapping software?

I already have the right hardware, good spec PC, tablet and pen etc.

Hoping someone will know the answer