Ginger,
When I was interviewed (earlier this year) I wasn't specifically asked whether I saw NATS and ATC as a long term commitment.. but many of the questions indirectly give you the opportunity to show that NATS and ATC is your long term career plan.
As you've pointed out, the fact that it costs NATS a huge amount in time and money to train an ATCO means it must surely be a fair line of questioning for them to go down. You may not be asked outright, but it's a good idea to have answers ready which show that you are committed to this career path and have examples drawn from your past that show you are able to stick to decisions/plans and follow them through. I.e. if you've changed jobs / university courses / etc a few times be prepared to explain exactly why!
Good Luck!