I assume we are talking about London and you have either made an appointment to take your documents and passport or have posted everything including the barcode you printed?
Non-Immigrant Type ED (To study in a basic education level (elementary, primary, and secondary school, or to study in a short course (Thai or English language) - single entry only)
Letter confirming enrollment details from school signed by authorised person together with a copy of ID card or passport of the signer
School’s registration certificate
Approval letter from concerned authority under the Ministry of Education
Financial evidence e.g. bank statements, proof of earnings, proof of sponsorship from a third party
Confirmation of legal residence in the UK or Ireland