Can a UK national enter Thailand from the Philippines without having an onward ticket booked?

You are allowed to enter if they don't ask. 😅. But if they (either your airline or Thai Immigration) ask you, and you don't have it, they can deny you boarding (airline) or deny you entry (Immigration).

I entered Thailand via Bangkok just over 2 weeks ago from Tokyo. Both the Tokyo check in counter AND Thai immigration in Bangkok wanted to see my departing flight.

It’s very rare that I (American passport) have been asked for an onward or return ticket. In the rare cases that have come up, most airlines, if pressed, will settle for you signing an indemnity document in which you agree to indemnify them against the costs of returning you home if the immigration authorities won’t let you in. As others have indicated, you’re even less likely to have this come up at the immigration counter. Only once in hundreds of international flights have I had to purchase an onward ticket. If you’re really worried about it and don’t want to commit immigration fraud, just tee up the purchase of the cheapest possible ticket to any neighboring country that you can enter without a visa. (VIA IS OK.) If it comes up you can make the purchase in seconds while at the check in counter.

Thai Immi seldom checks for onward ticket, however, the Airline check-in agent should be checking for onward ticket and that could be an issue.

Yes most airlines travelling into BKK from the SEA region are requiring onward tickets at check-in. You can get that easily online and then not use it for a nominal sum.

No

You need to ask your airline this question. Immigration isn't going to ask you to show your ticket out of Thailand but your airline might not let you board your flight without proof that you're leaving within 30 days.