I need to get my visas for China, Vietnam, Cambodia and Thailand by September and I don't want to risk doing it by mail and not getting my visa back in time before I travel. Can I simply go to each embassy in London and queue up and get them all done within a day?
How long will you be in Thailand? From the UK you will get a free Visa Exemption stamp in your passport at the airport. Good for 30 days. If you enter Thailand overland you only get 15 days free. So, if you have a UK passport and maybe proof of onward travel, that is one embassy you do not have to visit.
Some visas you can get at the border and some you can get from embassies in Bangkok too if you have the time. Each country is different. But usually Communist and former Communist countries you often pay for visas. Malaysia, Singapore and Philippines offer free entry with UK passport.
I'll be entering Thailand last and by bus, but I don't know when and how long I will be staying there so to be on the safe side I wouldn't mind paying extra for a visa.
I would prefer to get my visas sorted before I go and I don't mind queuing for hours to get them stamped. Can I go to the London embassies and do this?
"Can I simply go to each embassy in London and queue up and get them all done within a day?"
Every embassy will be different, but I would imagine that would take you more like 2 or 3 days. Have plenty of extra passport pictures, cash and your passport. Service is not usually instantaneous, and often they make you return a few days later. They usually have a process where you're checked against their "no-entry" list. I personally fly into a new country and use the easiest method - Thailand and Cambodia will be easy, Vietnam a little harder, and then there's China! Going the embassy route is normally more expensive.
I see that you want to apply visas for China, Vietnam, Cambodia and Thailand at the same time. If you go to Embassy, you can not do all within 1 day. You have to wait. If you make it online , you will see everything is faster. However, you should note that you go many countries at the same time, you should apply for multiple visa. I suggest that you should apply 3 months multiple visa first. With this visa, you can enter and reenter Vietnam more than 1 time during 3 months. From Vietnam, you can easily go Cambodia and Thailand, China. I ensure that visa on arrival is safte, not risky.