If you require a visa to train in one of the Foundation School then you will need a visa under Tier 4.

You should look at UKBA website
http://www.ukba.homeoffice.gov.uk/sitecontent/applicationforms/pbs/tier4generalapplication

The overarching sponsor for Tier 4 is UK Foundation Programme Office and you should read the information contained on its website.
http://www.foundationprogramme.nhs.uk/pages/home

If you are a Foundation trainee and you wish to apply for a Tier 1 (Post-Study Work) Visa or a Tier 1 (General) visa then you should read the appropriate webpage on the UKBA website.
See the links below
http://www.ukba.homeoffice.gov.uk/workingintheuk/tier1/poststudy/applying/
or
http://www.ukba.homeoffice.gov.uk/workingintheuk/tier1/general/applying/

If you need a letter from the Postgraduate Dean in support of your visa application then please e-mail img@londondeanery.ac.uk
In your e-mail please include the following information

1. Full name as it appears on your visa application
2. Your GMC number
3. Your address in the UK
4. Details of the Foundation School or training programme which you are currently on.