Whirlybird,
Not sure but perhaps you have two options with regard to the prize draw;
1. Find out if the charity involved would help you run the draw - they run the draw etc and all monies go straight to them. The resultant profit from the draw would count towards your minimum required.
2. The other I believe is to stop trying to run a draw and (like many magazines etc do) run a test of skill/knowledge whereby you accept a donation with each entry
e.g. "Guess how many hours this aircraft has been in the air for this year" Best guess wins a free trial lesson. Please make a £1 donation to XYZ charity with your entry".......small writing which details exact details of flight (weather limited etc) also includes information that a donation is not required to enter.
Something like that should get you arround the lottery rules and generate some money.
Regards,
DFC