First of all, it's not a simple question: you have to count the time you've spent on it, the cost of each tool you've used (pens, papers, colours...). While the price of the tools is easy to have (you just sum up everything you bought for the drawing and then do a rough 2 or 3% cost on it as a reference), the time you've spent on it isn't quantifiable as easily.
This said, I think that, for the tools you use and the average time you spend for each drawing (and counting the "real life issues" factor ), I'd say that 5.5/6£ are ideal
4-6 seems fairly reasonable. It's an affordable and fairly basic place to start. Perhaps if a lot of people seem interested in the comissions, it would be wise to raise it up a bit. Let's face it, more comissions means more work for you which is more time spent drawing which deserves, in my opinion, a bit more cash. Do what you see fit, my friend if you think the work is worth more, make it more and vice versa.