Technical backpack for day trips with backpack cover and internal compartment for the hydration bladder. Ideal daypack for excursions where we need a medium capacity backpack. The back with Air Flow System creates large air channels that will keep our back as cool as possible.
I don't know about Heathrow... but you can fly from London Stansted to Biarritz with Ryanair. And then bus and train to St Jean. However, depending on your schedule and money, it may just be easier to pay a little more and try and find a flight from Heathrow to Biarritz.