You would not get the £29.99 deal when you renew, but you will get some reduction.
The £29.99 is to attract new customers.
I've been with BT for over 20 years, I'm fed up with NEW customers getting better deals than loyal customers....BT are not the only company who does this !!!
Which is exactly why you don't get it.
More people stay with their present provider regardless of the price because they can be bothered to move or shop around.
If more people shopped around and moved to the best deal on offer rather than just staying with the same provider for 20 years there would be a need for ISPs to try and retain customers instead of the way things are just now.
At present for every customer that moves away there will be many more who won't so it makes no commercial sense to offer discounts to retain people when they are staying any way.