Two possible answers - the Statutory Minimum (SM) and the 'I'm a decent employer who does more than this'.
If SM
You are entitled to your contractual Notice Period.
You are also entitled to one week's pay for every complete year of service, capped at �330 per week. The multiplier of 1x becomes 1.5x if you are over 41 - check out these multipliers and less if under 22.
The killer is that �330 doesn't go very far to represent a week's pay.
The 'we do better' scheme
Employers can give whatever they wish in severance payments greater than this and many do. Armed with the above info, you can work out whether what you are being offered is better than the SM. There is no obligation to pay more (unless it is part of your T&Cs of employment - which is really rare to have included).
The point is, your question about how many weeks contractual notice you are on becomes somewhat irrelevant. Provided your employer is paying at least the SM, it's down to you and them to agree. I can confirm that every redundancy payments scheme I have been involved in negotiating IS based on actual years completed service - not contractual. Its fairer that way - the longer one has been there, the bigger the payment. The more one gets paid, the bigger the payment.
One final point - these schemes that offer more than the SM usually include agreement wording that you are being paid a severance which includes money in lieu of notice. Your employment terminates soon after the date of declared redundancy. This means your employer pays the severance amount without deduction of tax and NI (up to �30k). This is advantageous to you (it is also advantageous to the employer who doesn't pay employers' NI on it). Your P45 will have an earlier date on it - but you still won't be able to claim JSA until after the termination of NORMAL length of your contractual notice period.