For the Annex B, if you are using Annex B line and the VR400 don't work on your line, you may send an email to TP-Link Tech Support (support.uk@tp-link.com) to let them check it out for you.
For the 17A and 30A, it cannot be changed on the GUI and it is decided once the product is made. You can check the spec of VR400: http://uk.tp-link.com/products/details/cat-15_Archer-VR400.html#specifications
It says that the VDSL2 standards is Up to 17a profile.
And there are some models which support 30a, like VR200 : http://uk.tp-link.com/products/details/cat-15_Archer-VR200.html#specifications
Moreover, the 17a product would also work on 30a line, though the speed would be limited to the speed of 17a.

Do you know if the standards for the VDSL2 modems sold in UK are compatible in other countries and could be used as international version in the same way AVM has the German/Austria version and the International version?