Generally speaking, training to become an Alexander Technique teacher takes about 3 years. Most, but not all, teacher training courses are overseen by STAT – The Society of Teachers of the Alexander Technique. They are dotted around the country but most are located in and around London.
To find out about them best initially to log on to the STAT website – https://alexandertechnique.co.uk/ – and take it from there.
As I said, though, not all training courses are STAT affiliated. One that I thoroughly recommend that you check out is run by Delia Rosenboom in East Sussex. For more information go to www.southeastalexanderschool.co.uk