I'm just sending an invite to a friend for a Silver partner card and noted this.

It's definitely 1st September in the UK. Maybe only by 25 minutes but even so
I deleted and recreated the invite and get the same. Am I to assume that this part of BA.com works to GMT and not BST.....? Would prefer not to have to argue with BA later about it, so I suspect I'll now wait until 1:00am and resend it then.
<and an update>

So yes, uses GMT.