ACAS do arbitrate individual unfair dismissal cases. Often employment contracts from large companies will say "You agree to use ACAS for arbitration".

Large companies like this because it streamlines the process rather than dealing with different individual lawsuits, it also gives HR a number of policies to implement which can tick the boxes and give them a strong position in any dispute. If you have 1000+ employees eventually someone is going to claim unfair dismissal.

Smaller companies with 10 employees or less are more to just hope they don't get sued at all.

As far as I know you can't enforce arbitration / mediation in the UK it woudl not be justicable i.e you can't sign your statutory rights away

"Acas don't represent people in employment cases. Their job is to try and help you and your employer settle your case.

This doesn't mean that they will try to get the best deal for you, but one that both you and your employer will accept. So if you can't reach an agreement that both you and your employer are happy with, Acas will stop their involvement, and you can decide whether to go on with your case to the employment tribunal."

I think it would put an employer in a stronger position in court to say "employee refused to use arbitration service as described in contract", IANAL etc.

And the other side ah "so this illegal contract you have forced my client to sign" doesn't get your employers arguments off to a good start.

I don't see how it would be an illegal contract?

Not justicable you cant sign a contract to sign away your rights to annual leave - it's an open goal for the other sides barrister.

Not really the same thing as an arbitration clause unless you can persuade a judge that the arbitration decision would have contradicted employment law in some way.