i am a uk  resident  how do i start a charity in Uganda?

Call any scammer

You don't. But if you know people in Uganda, they can start a charity there and you can partner with them, helping with fundraising, project management, etc. 

I doubt you're british as they're too classy to ask this, check with passport office I guess

1) Form personal contacts with citizens of Uganda, (online or in person), until you find one that is trustworthy and knowledgeable. 'Vetting' requires contacting other reputable local charities or nongovernmental organizations 'on the ground' which are the best places out of which to 'recruit' someone interested in your ideas;

2) Present the idea for the charity online and begin interactions by using a website or having it 'hosted' by an existing charity's website;

3) Hone in on the 'type' of organization that best suits your vision from those that the Ugandan government permits and legally approves;

4) Establish a team of trustees and a 'sister' UK team, (ideally), including all relevant officers and begin to have meetings.

5) Plan a budget, funding sources and strategy;

6) Build good will in whatever community in Uganda your plans intend to assist by offering free services and assistance, including food or demonstrating a knowledge of customs and traditions and presenting your ideas with feedback expected from local tribal elders or community leaders;

7) Carefully document ALL of your activities, (pictures, videos, testimonials), and stay in constant contact with government or local officials while requesting new 'status' for your n.g.o. or charity accordingly, (there are many 'levels' based on ongoing approval), until all doubt about any negative intentions and opposition is fully quelled, all official documents are attained and nothing but a good reputation exists about you and your new organization.