The people with Infinity 1 with the speed boost now get the same upload speed as Infinity 2.
If I sold you a service for £30 with 80/20 speeds, and then tomorrow I sold Billybob down the road the same service for £20, and you wasn't made aware that there was a £20 service for the same speeds you'd be getting, I'd say thats a deceptive practice.
The law is based on the case law and the outlook of what is reasonable to the common person. In the example that would be a deceptive practice if it could be proven that BT knew of the price change while they were contracting you at a higher price and didn't inform you.
I would hope that BT will either reduce the price of Infinity 2 to match Infinity 1 prices, or scrap the labels of Infinity 1 and 2 and set a new price which lands in between both prices that would be charged to all new customers.
Though I'd still be paying slightly over the odds I can see there would be a genuine attempt to bring parity.
Demon => Freeserve => Pipex => Be => Sky => BT Infinity 2