Hey jov6,
There's no SLA for broadband as far as I'm aware, just phone lines. BT Business usually do aim to have any faults cleared by the end of the next day after they're reported, but this isn't always possible depending on the issue. It's best to keep in touch regularly with the complaint owner if you have any questions as they will be the best contact for your particular case as you've mentioned this has already been escalated internally.
Thanks, Bobby