It's very rare that I'll donate anything to charity. Ever. One time in recent memory was that I spent £12 on a £11.95 meal and put the 5p change in a bucket. This spans from 3 main instances with "Charity"

1. When I was younger walking through Glasgow with my brother, we were stopped by a lady from "Guranga". No idea what she was offering, but she said if I gave her £5 I could get a CD of the teaching of Guranga and a stress ball. I wanted the ball, so I gave her a pound. She refused to give me anything. I gave her £5 and she gave me a CD. She lied to my face bout the stress ball so I asked for my money back and she refused.
2. Roughly the same part of Glasgow, I was walking to the cinema and this guy walks up looking beaten up wearing dirty clothes holding a Coupon for McDonalds. He was asking for £1 so he could go to get some food. I refused and he looked distraught so I felt bad all day, even looked for him in the area an hour later but he was gone. Flash forward to a month later, and the guy was doing the exact same scam. Stopped us again. I was furious, and disappointed in myself for feeling bad, because everyone is a crook.
3. Again, Glasgow. I found £1 on the ground while walking to the train. A man walks up to me, we call them Jakies. Usually unclean, tracksuit wearing, drugged up etc. He was asking for £1 to get the bus back to Maryhill (a mile walk away, takes 20 minutes tops. I did it daily). Having just found £1, I gave it to him because why not. Seconds later, I see him sprinting down the street. Literally Usain Bolt style sprinting. He ran straight into the Bookies (Bookmakers) to gamble the pound I had just given him. To be fair I knew what he was going to do with the money, but it was hilarious to see him run for the bookies while I could still see him.

I have a strong dislike for "The Homeless" and "Charities" on the street, because most of them lie. I will probably give to medical charities / dog related charities in the future, but at the moment I don't. The trust is just not there having being lied to one to many times.

Edit - That being said though. I will tip well. People have offered me a service like Taxi Fair/Waiting on my table I'll give them a little extra. Usually +10% or rounded up to the nearest pound.