If you are a native speaker of English or have advanced English (minimum Cambridge Advanced/IELTS 7.0 etc) you can gain an internationally recognised TESOL/TEFL qualification by taking the Cambridge CELTA (certificate in teaching English to speakers of other languages) or Trinity Cert TESOL. These courses are externally validated by respected language boards and include observed teaching practice.
Courses are run throughout Britain in colleges of further education and some language schools and are taught at a level equivalent to a 2nd year undergraduate course (level 5 on the UK National Qualification Framework).
CELTA and Trinity can be done intensively over 4 weeks. They can also sometimes be done over several months or longer on a part-time basis. Course fees are in the region of £1,000.
You will come across very cheap online only ‘certificate’ courses but these are rarely recognised by reputable language schools. However, they could be useful as taster courses. Do take time to check that the certificate course you are considering doing is a properly accredited course because some have limited value. CELTA and Trinity courses can usually be relied on for quality and professionalism.
Both Cambridge and Trinity have diploma qualifications which are at level 7 or masters equivalent and often take a year to complete. They are suitable for experienced teachers.