What you may find is happening is that when you take out a fixed term contract you will probably have received a "discount" on the "normal" broadband package price because you agreed to a fixed term contract.
When this fixed term contract ends you lose the "discount" that you were given and the original full price of the broadband package then applies and it is this price that you would pay should you not enter into another fixed term contract.
This will invariably appear to be higher than a "new" customer would pay because BT show the price that a "new" customer will get when the discount is applied to the fixed term contract and not how much it will cost when the fixed term contract runs out.
If you have decided to renew your broadband package and enter into a new fixed term contract you will be given a discount on the full price of the broadband package for doing so and if you are not automatically given it you should ask for a discount and this should be on a par with what a new customer gets.
Check the small print "The monthly price for your broadband plan will increase each year from March 2022 by the rate of inflation (the Consumer Price Index rate, published in January each year) plus 3.9%. See bt.com/prices for details. All prices shown are in contract prices. Prices after 24 months increase by up to £8, if you don’t recontract".
