Squiffy - unfortunately, unless there is a company enhanced scheme, you don't get 20 weeks' pay.
The law on statutory redundancy pay is confusing for people because it talks about 'a week's pay per year of service' BUT it caps that pay at £310 per week. That equates to earning about £16,000 per year. Anyone earning more than that does not get "a week's pay", they get £310.
The complication that the amount increases to £460 per year for years of service aged 41 or over. Obviously, to get to 20 years of service, chances are that you will be 41 or older. Depending how old you are, you can therefore be earning more than £16,000 and still get a full week's pay per year under the statutory redundancy scheme.
If your suggestion is a defined period of 'working' whilst looking for another job and then payment, that sounds like a good suggestion for the OP. I am sure that the company would want a definite committment and compromise agreement though. They wouldn't want a situation where, at the end of the four months, you could turn around and say "actually, I haven't found anything so I'm staying put".
Also, severance payments are based almost solely on financial loss. If the employee can find a new job just as good in four months then that (less any paid but unworked notice period) is all he would get if he sued, no matter how unfair the dismissal. That might make an employer a bit loathed to pay a whole year.
I think that actually, a year is a good deal if the individual is confident he'll get another job. If he's not (particularly if he's older and concerned he won't find anything ever again) then I'd be inclined not to agree to a package at all.