If you have kept within the terms of your overdraft and there is no reason for Lloyds TSB to review the account, then I would suggest that you write back in response to one of the letters, asking them for the exact purpose of any review. Point out to them that the last time they did a review they tried to hard sell the banks products, even though it was made clear that you did not want that to happen.
My understanding is that ANY bank can review ALL account terms/conditions and withdraw any of the facilities. So it would not be unique to you. There are some rules regarding fairness and causing hardship which they would have to comply with. So they could not suddenly withdraw any overdraft, if that was to your detriment. They would have to give you notice, but I am not sure of the timescale.
If you called Money advice service helpline, I am sure they would provide more info. http://www.moneyadviceservice.org.uk/yourmoney/