As you would no longer be an employee, your former employer would be entitled to bar you from entering the premises in order to speak to your former colleagues.
Of course, outside of its premises, your former employer cannot prevent you meeting your ex-colleagues. Bear in mind though that, as existing employees, they have a duty of trust and confidence, which prohibits them from disclosing confidential matters. Also you as an ex-employee, under your contract may owe a duty of trust to your ex-employer, such as (for example) to prevent you from disclosing trade secrets.
