I do not think they will qualify for early entry into Thailand at this time.
And the process to obtain permission is thru the UK Thai Embassy. Takes one to two months to gain permission if eligible.
Followed by costly one way flights from the UK to BKK on repatriation flights only.
Followed by 14+ days in mandatory quarantine in an approved Bangkok hotel.
I suggest you delay the family reunion until a much later date in late 2020 or early 2021.
Good Luck.
If you wish added information suggest you follow this UK Facebook group of expats seeking to enter Thailand.
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