Hey Anne
I deal with this a fair bit but before i say anything I would point out that the decision your taking is a very personal one and wholly dependent on your circumstances now and expected circumstances in the future.
just to correct your initial statement, you are not actually taking the lump sum now, you are waiving your right to it which has the result of increasing the amount of SCSB relief you can use, thereby decreasing the taxable amount of your severance package and increasing your bottom line.
Like i said, this is a personal choice, a lot of people look at the difference in in the SCSB relief figures as "the cost of keeping your options open" i.e. you cant touch the pension until you're 50 ANYWAY at a minimum (thats why i corrected your statement) so if you're likely to take up more employment, then at your age you have ample time to build up another lump sum in another pension.
If on the other hand you dont intend working, then the lump sum may come in handy when you reach 50/65 (depending on the terms of your pension) but based on my experience (and this applies especially to younger people) if you waive now, you probably wont even realise you did it in 20 years time (again based on the premise you'll be working in the future)
whatever option you take will still result in an annual pension entitlement (can reduce slightly depending on what option you take) but the main deciding factor for now is do you/need or want in the future the lump sum element of your pension.
hope i have been clear.
If you want any other information, please let me know.