Mrs. O
Your hubby's experience is not untypical of a less-caring company's approach, regarding the 'it will be worth less money to you if you reject the offer on the table after a job trial'. Some companies will maintain the same offer. See here about the 'rules' on trials.
http://www.direct.gov.uk/en/Employment/Redunda ncyAndLeavingYourJob/DG_10029844 (scroll down a bit)
My suggestion is that it is not worth fighting the argument about less money later now. If your hubby would prefer the job (to redundancy), it is better to go into the thing with the company believing his commitment to it, and then in the event that it doesn't work out, THEN try fighting the argument about it is worth less money at THAT time.
If you look at the link you will see that the legislation says that the employee shall not be in a worse situation from rejecting a trial than if he accepted it in the first instance. But that is against the statutory redundancy amounts. If the employer is offering enhanced terms for redundancy now then I suspect it may be within the law to modify those enhanced terms in the event of a rejected trail period (I am not sure - this is a fine point of legal detail and I am not a lawyer). The other way to think about it is that your hubby will have had a longer period of employment in total if he goes into a trial that he later rejects.
Keep records of discussions he has on this - he may need them later if the worst happens (e.g. he goes into the trial, rejects it, is paid smaller sums and decides he wants to fight that decision through the legal process).