Can my employer prevent me from speaking to my ex-colleagues when I leave?

As you would no longer be an employee, your former employer would be entitled to bar you from entering the premises and speaking to your former colleagues.

In other circumstances your former employer would have no right to prevent you meeting your ex-colleagues, but, as existing employees, they have a duty of trust and confidence, which prohibits them from disclosing confidential matters and trade secrets.

As an ex-employee you too may still have a duty of trust, extending, for example, to a prohibition of disclosure of trade secrets.