If you are at the end of a 12 month contract, then any special deals would have ended, so you will have to negotiate a new contract by calling 0800 800 030.
As you are talking about the new customer deals, then these offers vary from month to month, as do the voucher amounts.
This has no effect on existing customers.
If you enter into a contract with BT, then if they increase the price during the contract, then you can leave without penalty.
You are not seeing out of contract prices on the BT website, you are only seeing new customer prices, which as I said, vary from month to month. There will be special deals from time to time, like the Black Friday offers that were quite a bit cheaper.
You also have to take into account any variation in cashback of voucher offers.
As far as I can tell there does seem to be a slight increase in price.... if you look in the small print at BT's Infinity 1 (with unlimited weekend calls) pricing structure it's showing at £47.49 at month 13 (full price - no offers). I'm very near the end of my first 12 month contract and my 'upgrade deal' is £49.99, so why the extra £1.50 ? (BT's advertised full price is showing at £49.99 on their main page so you'd need to look further in their tariff guide where they have all of their call charges etc listed)
A few weeks ago I was offer a reduced deal to renew which appeared on my online account. It was about £5 more than I'm paying now but it's been removed and has been replaced with the full price offer. I didn't accept the reduced price because I'm not in my last 30 days until next weekend.... so why so early ?
But going back to the OP, I think he is right about the increase. Can any BT mods clarify this without telling us to ring customer services ?