I've stuck a timetable together for the Flixbus UK operations and will (attempt to) keep it updated as the schedules change and expand.
Initially they're running 3 routes from London - to Portsmouth via Guildford, Bristol and Birmingham.
Services run Thursday to Monday initially, and will be operated by two operators, BM Coaches of Hayes and Turners Coachways from Bristol. Vehicles are expected to be in the distinctive green Flixbus livery. I've used Flixbus in Europe, where they run a huge operation and they're a typically well organised German run operation so expect routes to be expanded quickly in the UK as we travel more and more.

Schedule attached below...