The law says you must not be forced to sign an opt-out, or treated unfavourably ('suffer detriment' in legal jargon) if you refuse. Even where you have agreed to sign an opt-out, there is nothing to stop you reversing your decision later.

If, as a result of rejecting an opt-out, you are treated unfavourably or even sacked, you can take a case to an employment tribunal. Seek advice from your union or an advice agency such as Citizens Advice.