The law says you must not be forced to sign an opt-out, or treated unfavourably (‘suffer detriment’ in legal jargon) if you refuse.
If 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.
The law is less helpful if you are asked to sign an opt-out as a condition for starting a job. Strictly speaking this probably should not happen, but there is little in practice you can do about it unless you are in a union.
However there is nothing to stop you reversing your opt-out at any time, and you are protected from unfair treatment.
In practice if you are sacked, you will have a stronger case if you have been in your job for more than a year. This is because after a year you gain protection from unfair dismissal. Specifically, you will be entitled to know why you were dismissed and to challenge the employer in the tribunal, as to whether it followed a fair procedure in dismissing you.
While an employer cannot sack you (or discriminate against you in any way) simply for exercising your right to reverse an opt-out (even if you have worked for less than a year), it is harder to challenge the employer when your service is less than a year.
