I don't know if it is definitely the 15th (I believe it is, mind), but do we need a new thread to give a date? The changes, and that date, have all been discussed before in the other one.
We don't create a new thread every time someone finds a new place that has 4G coverage do we? It goes in an existing thread.
And as for the tethering allowance, I've been told a different amount which isn't the same as the handset tariffs. There might also be some additional (time limited) promotions on one or more of them, presumably to entice people to sign up or possibly even upgrade, if things like free 0800 calling is of interest, and tethering isn't deemed vital (beyond the amount included, with the ability to 'top up' the tethering allowance every now and then).