Found this - although it is quite old
here. Think it means that provided all you got was statutory non taxable redundancy (less than £30k) then does not have to be declared on a tax credits application. But I try and find something more up to date!
On the Revenue site here it says that if you stop working your Working Tax Credit stops after 4 weeks. Presumably this will then be replaced with Jobseekers Allowance if you are looking for another job - so you would have to check if that is affected by a redundancy payment. From this it looks like it is a fixed weekly amount and not means tested.
Maybe you could try talkinig to somebody at the Jobcentre for some advice about what would happen if you took the redundancy and started looking for another job?
Sorry if this has duplicates anything already said - started typing reply then had to go deal with nappy change!