....Can the free BT 12 months of XBox Game Pass be linked to a new/ie. a separate XBox Account....
Well I can partly answer my own query.
The BT free 12 Month Gamepass System is sub-optimal in that BT do not just send you an Xbox Gamepass Code, (like the Xbox Codes you can buy from Amazon), BT have invented their own BT controled system via which they can take over the payments to your Xbox Account if you sign up to it.
To obtain the free 12 months gamepass offer you need to sign up to the BT Gamepass ADD-ON via your MyBT and by doing so you effectively give BT control over the management of the Xbox Account, that BT is linked to, at a cost of £10 per month which is free for up to 12 months which is definitely not what most would want.
Therefore having done some serious googling about the way in which the BT free 12 months Gamepass Ulitimate operates some of the answers are;
(1). When you opt into this BT Add-On: BT takes control over the Microsoft Xbox Account that you sign up with, (at least with regard to account payments to Xbox), and when you opt out it can take a long time for BT to release control over your Xbox account so that it can revert to your control and the previous payment/pass record.
(2). If you already have your own Xbox Ultimate Gamepass linked to your account, your own Gamepass put onto a back burner and is inaccessible while BT have control over your account payments and when BT eventually release your account, theoretically, your old Gamepass should return unharmed.
(3). One work around is that you can keep BT away from your usual Xbox Account by linking the free BT 12 Months Gamepass to any Microsoft Xbox Account that you have control over. - Therefore, to keep BT out of/away from your own Xbox account, (which in my case is tied to my main email address), you can set up a new/a dummy account for use by you or by a family member. - Not ideal but it is a workaround.
(4). Alternatively allow BT to control the payments to you main Xbox Account until they release it back to you.
Personally, I plan to keep my main Xbox Account separate and well away from BT by setting up a new/dummy Xbox Account just for use with the BT Controlled Gamepass. - It may be useful to a family member or grandchildren, (subject to them keeping up with their schoolwork and not spending too much time on the Xbox).
When I signed up to BT the free 12 Months Xbox Gamepass was not a main reason to join them but it was a consideration and this system not what most customers would want.