The Annex refers to whether the broadband line can provide an asymetric service - unequal upstream and downstream speeds or a symmetrical equal upstream and downstream service. For ADSL in the UK only Annex A is provided with a suffix of Annex M for increased upstream bandwidth.
For Openreach FTTC in the UK it's theoretically Annex B even though the line is 'legacy configured' with a tone set for an asymetric service Annex A. Just set the router to Auto. For a router that complies with the Openreach specifications for connection it shouldn't make any difference whether it says Annex A or B for your FTTC service. Routers are supplied for the pan-European market so include these different Annexes for compatibility across Europe.
Range A is the speed range for a near perfect line with minimal impairments from noise and crosstalk, whereas Range B is the Openreach speed range estimate of what you might expect if there is crosstalk, high background noise or imperfect wiring.
g.Fast is being rolled out by Openreach but not all locations where you see the cabinet side-pods are being trialled by TalkTalk even though the Openreach publicity says TalkTalk are a triallist. Here's the information you need for g.fast from TalkTalk. It all seems to be quite low-key at present.