I wouldn't get too hung up at this stage about which start time you have managed to get as it's all likely to change if the Brexit business isn't resolved by then and we are still EU members.
It seems that ACP are planning to move all of the British riders into one start group, which will be sent off first, ahead of the vedettes. Riders will be able to ride to the first control at Mortagne, but must then turn around and ride back to the start. They may then be given permission to rejoin the event and continue to an as-yet-unspecified 'final' control somewhere on the return leg between Brest and Loudeac. They will be allowed to abandon the event before reaching this control, but must give two controls' notice of their intention to do so. If they do choose to continue beyond the 'final' control, they MUST complete the event, no matter how long it takes them. However, the controls will not stay open for them and they will have to negotiate their own arrangements for food and shelter along the way.
As an alternative, British riders will also be allowed to remain in their currently allotted start group but will be required to spend 90 hours riding around the velodrome at Rambouillet and, naturally, will not have their rides validated.