Please see below, I have copied this info from UK Border agency website:
Your application must include details of two referees. The referees should have known you personally for at least three years. One referee should be of professional standing such as a doctor, minister of religion, civil servant or a member of a professional association such as an accountant or a solicitor.
See the list of Acceptable professional persons.
http://www.ukba.homeoffice.gov.uk/sitec ... er6/annexa
The other referee must be a British passport holder. Both referees must be:
* aged 25 years or over; and
* not related to you; and
* not related to the other referee; and
* not your solicitor or agent representing you with this application; and
* not employed by the Home Office.
The referees must not have any unspent convictions.
Lastly, just to emphasize, 1st referee (professional) doesn't have to be a british national
