Do I have to resign to claim constructive dismissal?

Yes. The claim depends on your resignation in response to the breach. Until you resign there is no dismissal to complain about.

How long can I wait before resigning?

Not long. Delay suggests you accepted the breach. If you need time, say in writing that you are working under protest while your grievance is considered.

Do I need to raise a grievance first?

It is not a legal requirement, but a grievance records the breach and an unreasonable failure to follow the Acas Code can affect compensation.

Can I claim if I have under two years of service?

Only where the reason is automatically unfair, for example discrimination or making a protected disclosure. Otherwise two years of service is required.

What is constructive dismissal?

Constructive dismissal is resignation in response to a fundamental breach of contract by your employer. The breach, the response and the timing all have to line up.