That's absolutely right, F30, - and perhaps I should have made this clear - taking on a 'suitable alternative role' is an option to taking redundancy. You will not get both.
However you are legally entitled to a trial period of four calendar weeks in the 'new' job. This is for you to assess the job and if you resign within that time you are legally in the same position as if you had been offered the job but declined it from the outset.
However you need to be aware that if you decline an alternative position, whether during the trial period or whether at the outset when first offered you are still entitled to redundancy unless two criteria apply. These are:
The alternative employment was suitable
Your declining it was unreasonable.
This is deep in legal territory and I am not giving you specific advice, however, suitability is assessed based on whether the package of pay, hours, role/responsibilities, duties is broadly similar. The pay can be a bit less, but what you are being offered sounds too far less - if push came to shove at an ET.
It doesn't stop you trying it out though, at no (apparent) disadvantage to yourself - to bide for time.
Try and get to the CAB to get referred to a solicitor for 30 minutes of free advice, if you can get it.
At least you know something of the basic questions to ask an expert.