Can I change from part time employed to self employed?

[Yessa]

I currently work for a small charity on a part-time basis (one of 2 part-time employees, I manage my own time and workload). I get paid approx £9,750 per year (before tax) which is supposed to be for 18 hours a week. I find myself volunteering above this and end up working 25 - 30 hours per week. Most of the work I do at home using my own computer. I don't have a contract, only verbal agreement. I also want to run my own small business (market stall selling craft goods once a month) whilst continuing to work for the charity on a part-time basis. Next year I am considering having a baby.

What I'd like to know is can I change my status to self-employed, and then invoice the charity once a month for the amount they already pay me? As I understand the situation, this will mean I can then write-off petrol and other expenses, as well as earn a small supplementary amount from my market stall. I also have heard that the maternity pay for self-employed is higher than employed?

I'd be grateful for any suggestions and help!