Do I need an outward flight from the UK to apply for a 60-day tourist visa to Thailand?

A Tourist Visa is valid for a single 60 day entry to Thailand up to 90 days from the date of issue so don't apply too early. The Evisa syatem and the London Consulate normally issue the pdf Visa in about 3 days.

You will make your application using the Evisa online application system.. and MUST show PROOF of your flight departing the UK to Cambodia then to Thailand... show both of the flights.

Just how long are you planning on staying in Thailand and the SE Asia area THIS trip?

Generally it does. You can try submitting an itinerary. That basically means go into an airline or booking website and find a flight and go all the way to the end where you can see the details but haven't paid yet. Then print that to a PDF or a JPG and submit it with your visa application for flight information.

If they do not accept it they will email and then you look into using an onward travel website to get temporary proof.