Could anyone tell me if it possible for a Thai national to enter the UK on a tourist visa with a one way ticket, or must they have a return or atleast an outbound ticket? Cheers...

There is no requirement for a visitor to have an onward ticket, but expect a few questions from the immigration officer on arrival.

Scouse.

If the Thai national is asked to show their return ticket on entry (which is highly likely) and they have none, they should be prepared to be taken aside and quizzed about their intentions. They had then better have sufficient proof with them to show when and how they will leave.

My wife did that once and after a grilling they allowed her in but restricted her visa time to the period she said she would be in UK (which then raised questions when we applied for the next visa)

Get her a return ticket with an open return on it, book return 1 month before her visa runs out, saves helluva lot of hassle either end,