@Paul *****
If you want to stay for 90 days you apply online for a 60 days tourist visa and do a 30 days local extension at your local immigration in Thailand as explained. These are the general requirements and you apply online by creating a user in the e-visa portal.
Based on feedbacks I know the embassy in London issues tourist visa with onward flights after the initial 60 days, and I recommend that you contact your airline and ask if this also are ok with them. At the immigration in Thailand it’s normally no issue.
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