http://www.acas.org.uk/
Have a look at the ACAS site for info. I don't think that there is a time limit, but they must act reasonably. Constructive dismissal is very difficult to prove, and at such an early stage, I wouldn't even contemplate this step.
Have you received a copy of your company's disciplinary code of conduct?
Are you in a union? Ask your rep for advices. If not, do you have someone who you trust who could attend the next meeting with you, to act in support and maybe take notes, as you are allowed this.
If none of the above apply, make an appointment with your local CAB, as they can advise and support on these matters.
Finaly, check you household insurance, as many contain legal expenses cover that can be applied to employment disputes, should it come to that..
Keep us informed as to your progress, and good luck.