Good luck with that, sounds like a worthy cause.
How about this...
1. Write up the event in words, describing the plan. E.g. 20 Mile night right around Hastings in aid of XYZ
2. Get some help putting this all together, make a poster or flyer, set a date, time
3. Put a simple website or Facebook page together
4. Contact the Council and make them aware of your plans (I don't think this will be an issue and they may even help)
5. Sort out a Fund Raising account via one of the online vendors.
6. Organise how people can get on-board, how they register to take part etc.
I imagine some insurance policy would be a good idea to cover the organiser.
Question for you - Is the Drop in Centre a Registered Charity ?
Another way of getting support would be to use the Charitable status to gain places on other organised rides and events, so you don't have to organise one yourself - just an idea.
Seems quite a long list and I don't want to put you off.
I have never organised anything like this, but I have done several charity rides and have got to know some of the charities well in doing so.