Be clear. Where was the mother born?
If born in the UK ('a British otherwise than by descent') then YES, the mother can pass on the citizenship to the child and the child will have a British citizenship ('a British by descent')

If born in Thailand ('a British by descent') then NO, the mother cannot pass on her citizenship to the child.

In other words, the mother of the child must be 'a British otherwise than by descent' and not 'a British by descent'.

http://www.morton-fraser.com/knowledge-hub/british-citizenship-children-born-abroad