I have the same enquiry as you have. Below is the answer from HO.
Dear Sir
Thank you for your enquiry.
If you wish to study full-time you will require a student
visa.
As you are from a visa national country you are not
able to switch into the student visa category whilst in
the UK.
You are required to obtain entry clearance as a student/
prospective student, regardless of the level of course you
wish to study, at the nearest British Diplomatic Post in
your home country.
You will need to contact the nearest British
Diplomatic Post in your home country for further information.
The Home Office does not issue Entry Clearance.
This is the responsibility of British Diplomatic Posts around the world.
Alternatively, you can contact UK Visas on 0207 008 8438 or visit their website at
www.ukvisas.gov.uk. Their email address is
Publicvisaenqs@fco.gov.uk