There's no doubt that there are advantages to AVCs and disadvantages to nostional service purchase. However, some years ago, I was speaking to an AVC seller (probably Marsh financial services) and outlined the specific thing I wanted to fund for - essentially a buy-back of years so I could retire on full pension at 60. The scenario I outlined was (in my personal opinion) a text-book example of where NSP is ideal; what I got was a hard sell on AVCs which would have been substantially more expensive, and not a whisper of a mention of NSP. I ended up not arranging NSP for about a further five or six years, and despite my now higher position and salary it's still around the inflation adjusted cost of the AVC recommendations I got back then. Had they pointed me in the direction of NSP at the time, I'd have started buying back those years straight away and it would have cost me significantly less over the years - and I have a lot of years left to retirement!
Point is, I suppose, that sales people are going to extol their own products and often to the exclusion of other options. I would now recommend that any person seeking advice on pensions, lump sums or retirement do a lot of research - and if they aren't equipped to do it, then to get proper, independent, financial advice. Several unions recommend companies who present themselves as advisors but who are absolutely not independent.
I think those recommendations, issued without proper health warnings, do the members a real disservice.