e-visa for Cambodia

I didn't know you could get a visa for 3 months to Cambodia. Do you know if this applies to citizens from all countries (UK here) or just select countries?

Also, if doing one of those visa runs in a mini bus, just going to the border to activate my Thai tourist visa, do I have to get a Cambodian visa? I am hoping not but guessing I will have to pay something when I get there!

Thanks