Quarantine if a colleague diagnosed?

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If we've potentially been exposed to coronavirus in our place of work, are they guidelines about self isolating?

No, there arent.

No, you don't need to isolate. Only if someone in your household has symptoms.

www.bdbf.co.uk/coronavirus-and-the-workplace-twelve-key-questions/

One of our employees has been diagnosed with the coronavirus – what should we do?
The employee will either be treated in hospital or cared for at home. The employee should not attend work until they have fully recovered and have medical clearance to do so. You should pay sick pay to the employee in the normal way (see 9 below).
You should notify other staff that a colleague has been diagnosed with the coronavirus, however, you should not usually need to name the individual and you should only disclose necessary information. The Information Commissioner’s Office has published some short guidance for employers on this issue.
You will be contacted by Public Health England’s local Health Protection Team to identify those who have been in contact with the employee and discuss any special measures to be taken. This may affect the following staff members:
anyone who has been in close face-to-face or touching contact;
anyone who has talked with, or been coughed on, for any length of time while the employee was symptomatic;
anyone who has cleaned up bodily fluids; and
close friendship or work groups.
They may advise that anyone who had contact with the affected employee should self-isolate for 14 days and be tested for the virus. They will advise you how to undertake a deep clean of the premises and how to collect and store any rubbish produced by the affected employee.
Staff who have not had close contact with the affected employee do not need to take any precautions and can continue to work.