Definitely 2nd tier. Very British. I doubt they have any Americans/Canadians working there. They also do not hire locally; at least that's what their ad has said in the past. Interviews take place in London only. Probably around 100k for salary.
I have it on good authority that the 'real' international schools in Thailand (that would include Harrow) take care of the thai teaching license issue in-house with little to no involvement from teachers.