Beyond the pilot, BT needs a vendor that can give them 48-port and 96-port nodes, which don't exist yet. I'm surprised they'd want to lock down on vendor choice just yet.
BT had a 'Investor Day' (webcast on their site) recently where they briefly mentioned they had chosen a chipset for their 25,000 pilot but as you say they will want to keep their options open.
I believe 96 port nodes should be available by the end of the year.
As I posted above, I understand that Alcatel and Huawei units are using Broadcom chips and Adtran Sckipio chips. Broadcom's chip supports VDSL as well as G.fast. Sckipio's chip seem to perform better better but some telco's may want the flexibility of continuing to support VDSL.