Hi half
On the subject of NHS antenatal classes versus NCT classes, from what I gather the things that are different about NCT classes are:
Each set of NCT classes is run by an individual who isn't following a set 'curriculum' so what is taught can vary between different courses according to the personal opinion of whoever is running it.
People go to NCT with an expectation of forming a useful friendship group so that you build an instant support network of people who will all be having their children within weeks of each other and will have children the same. This can be invaluable especially in the early days when everything seems overwhelming.
NCT membership gives you access to 'nearly new' sales etc.
The NCT classes cost a bomb (my local classes work out at nearly £50 per one evening class).
That about sums it up from my point of view 
I can't justify the cost and also don't feel the need for a set group of friends. I don't have much spare cash and have decided I would rather spend £250 on going to baby groups, swimming classes etc. once the baby has arrived. I hope/assume that I'll strike up some friendships at these things.
In terms of preparing for labour, I think the antenatal classes my local hospital run should be sufficient and I'll do some reading if I feel there are still gaps afterwards.