They usually keep a couple of loaners for emergencies. Since this is a routine/ non-emergency appointment, there's no reason to fret. The 1200 mile service is +/- a few hundred miles. Unless you plan to put 1K+ miles on your car before the 28th, I wouldn't worry about it.
Current: 2009 E90 M3 | SG | FR | 6Speed | Akrapovic Evo | JRZ RSONE | StopTech Trophy | Cantrell Brake Ducts
On order: 2016 F80 M3 SO | AB XT1 | ZCP | etc.