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. You may also, as an ex-employee, owe a duty of trust to your ex-employer under your contract – for example, not to disclose trade secrets.
If you have signed a settlement agreement under which you have left your employment on agreed terms, it is not unusual for the agreement to include a promise not to disclose the settlement terms – especially the settlement sum – to anyone except your immediate family and your legal adviser. It is very important to keep this kind of promise. If you do not, and your employer discovers that you have told your ex-colleagues how much they have agreed to pay you, they are likely to be entitled to withhold the settlement payment.