Is having a return ticket to the UK for February acceptable for entry into Thailand after a 45-day waiver?

A traveler from the UK is asking if having a return ticket to the UK for February is acceptable for entry into Thailand after using a 45-day visa waiver and then traveling to Laos. Comments indicate that while immigration is unlikely to check this detail, airlines may not allow boarding unless proof of onward travel is provided, preferably within the 45-day period. It is also suggested that a ticket to Laos could suffice as proof for the initial entry.