Jethro's shows no planned date for NLL to be withdrawn from use. There are, however, quite a few scheduled for withdrawal in 2014. It might be that BA has some juggling around to do, as most of those being withdrawn will likely be due a major imminently.
I note that NLL has been at CWL twice in the last 6 months for maintenance, 3/6 - 13/6 and 12/8 - 25/9. The latter 6 week stint would suggest that a major check has been recently performed.
I further note that it was due to get a first class refit from one of the aircraft due to be withdrawn in the next few months, suggesting that NLL was scheduled to be operated for 2 - 3 years yet.
For BA, it may be a question of the cost of repairs to NLL vs the cost of a major check on another aircraft due to be withdrawn.