Double counting the notice period is unfair IMO. You already pay for the notice period by having the notice period when you want to quit. Companies having to pay you the notice period when they fire you is symmetrical and IMO is not a severance package. Maybe that's just my assumption, but IMO a severance package is the asymmetrical pay-out when one party quits an agreement, not the communal wager you both put down when you enter the contract.
Put another way: if both parties agree on a shorter or longer notice period, I wouldn't expect that to affect any potential severance package. It's just the notice period.
We're talking about worker's comp here. In this thread specifically, the UK is brought up as having generally "better" severance packages. But that's only half the story if you count things which the workers pay the companies when they're the ones quitting.
I worked in the UK, I've had to "pay" for that notice period by hanging around where I didn't want to. It's the other side of the coin which somehow doesn't get mentioned when people bring up Europe as somehow having better employee protections. They might, but notice periods ain't that.
If I had to put as much money into a company's retirement as they put in mine, I wouldn't turn around when I retire and say, wow, great comp package. No: this was a symmetrical deal we made, this time it's working out for me--in a parallel universe it's working out for you; it's a wash.
Severance packages are comp. Notice periods are just properties of the contract. They're not a severance package.
I find this an important distinction because it lets companies pull the wool over your eyes by pretending they're being generous, when really they're just paying you the exact same thing you'd have to pay them were you the one quitting. That's not a package, that's just salary.
Working my notice period has never be even an issue for me. It gives me an opportunity to wrap up projects, say goodbye to colleagues, etc. It's usually fairly light work as well, you're not taking on new responsibilities. I didn't even realise it could be otherwise.
The reason I'm saying it's part of the redundancy package is because (some?) companies will pay off your notice period without you having to actually work it. I've taken voluntary redundancy only once, and I was told that I would stop working at the end of the month, but I was still paid my full 3 months of notice (in addition to the tax-free redundancy payout). That was not part of the initial contract.
Put another way: if both parties agree on a shorter or longer notice period, I wouldn't expect that to affect any potential severance package. It's just the notice period.