Speaking as an ex plod (now an ATCO), this has been discussed in detail previously on the ATC forum.
Basically, until new legislation s passed through parliament, which will define a specific limit and the powers of authorised persons to require a sample, then they cannot legally require you to take a breath test.
This does not preclude companies from exercising company policy, but it's a legal minefield at the moment, especially with the advent of the Human rights legislation.
Random testing would be an infringement of human rights, you could only be tested if an authorised person (probably a police officer) had reasonable grounds to suspet you had been drinking (i.e could smell it on your breath), or after an incident whereby safety was compromised.
If it was the former of the two, and there was no alcohol at all found in your system, then you have reasonable grounds to sue the @rse off them and make a killing!!!
Hope that helps!!!
------------------
Turn left heading 230, close from the left, report established