Hello! That sounds like a great adventure through some magnificent places in France. Here are some tips to help you on your journey.

Best Time to Visit:
The best time to visit Biarritz, Pau, Sarlat, Bergerac and the surrounding areas in France would ideally be from June to September when the weather is most favorable. However, keep in mind that summer months (July and August) can be quite busy due to European holidays.

Route and Accessibility:
You can start your journey in Biarritz, which is accessible through the Biarritz Airport. For Pau, Sarlat, Bergerac, you could either choose car rental or consider using the extensive train and bus services that France offers. Be sure to check the schedules and plan your travel ahead of time.

Accommodation and Booking:
You will find a wide range of accommodations from luxurious hotels to cozy bed and breakfasts in these areas. It's best to book in advance especially in the peak tourist season. Reputed online portals like Booking.com, Airbnb, or the local tourism websites could be used for bookings.

Clothing and Essentials:
Pack airy and comfortable clothes for the summer months and be prepared for occasional rain showers. If visiting in the cooler months, carry warm clothes. Essentials include sunblock, a reusable water bottle, comfortable footwear, and a good-quality camera if you enjoy photography.

Health and Safety:
France is generally a safe country to visit with excellent healthcare facilities. However, don't forget your travel insurance. If you are a EU resident, avail EU Health Insurance Card before travelling. Regular precautions like mindful of your belongings, avoiding deserted areas at night, etc should be taken.

Local Etiquette and Respect:
The French appreciate polite behavior. Basic phrases in French such as 'please', 'thank you', and 'excuse me' will take you a long way. The locals also appreciate an attempt to speak the local language even if it is not perfect.