score:2
The main border crossing for buses is Moc Bai - Ba Vet. That's most probably where you'll cross if you take a bus. The best way to get a Cambodian visa is to get it online. Print two copies (immigration keeps one). This limits the ways you can get scammed at the border. There's nothing else to pay, whatever they might tell you.
On the way back, you won't need a visa. You will transfer airside in Tan Son Nhat. The airline will (most probably: we would need to know exactly what the airlines are, but my experience here says it should be ok) chexk you in all the way to London.
One point about your passport and Vietnam: your passport (like mine) allows you to get visa-free entrance, with a 30-day gap. If you need to enter Vietnam twice, you can apply for an evisa. You wouldn't need a multiple-entry visa.