I've just been through a restructure with work and looked in to this scenario as they were changing the job title and description of everyone in my role and didn't need as many of us.
During our consultation I spoke to ACAS and a very senior HR relative, both said the same thing. If the company has a role for you at the same level / pay that you don't accept you can be deemed to be making yourself voluntarily out of work so not obliged to pay redundancy.
In our case they decided not to force any one into this role so if you volunteered for redundancy they would allow you to go, even if it potentially left them with vacancies. It's not uncommon for companies to do this where there's a significant change in role, but legally they don't have to. It's a little bit different if they're asking you to apply for a new role rather than just offering it though.
There does have to be appropriate consultation regardless, and the numbers involved effect how long that is (I think 20 makes it 3 months rather than 1 month but I'd have to double check that).
Worth pursuing the opportunity for redundancy and if they agree then they may still try and persuade a trial period in the new role, but the agreed redundancy terms should still be honoured if the role is not taken up permanently. The key is to have the agreement that this is a redundancy situation and a severance agreed as you wouldn't be able to take the job, decide you don't like it and then try to persuade them to make you redundant (well, unlikely to be successful is more accurate, you'd effectively be handing in your notice so no need to pay redundancy).
Again they've done this to plug the gaps at my ex company until they recruit, offering those that have accepted redundancy a chance to work in the new role and gain the experience for 6 months but still walk away with the redundancy package after that (or earlier if they decide to). This is a bit unusual and only because it suits the business to minimise vacancies and have someone experienced in post to hand over to new starters.