In Mystery on the Moor, Doctor Alton Bentworth was a member of the Longborne Flying Club and based his Auster there. He used the Auster for his daily commutes from London to the farmhouse on Dartmoor where he was conducting plastic surgery to change the appearances of criminals so that they could evade the police.
On the morning of Biggles' planned raid on the farmhouse, he wanted to be sure Bentworth was on the scene before he moved in. Accordingly, he flew to Longborne and then followed Bentworth's Auster as it made its way to Dartmoor.
The location of Longborne was never given but considering that Bentworth, a Londoner, used it for regular commutes, the club must have been in London. Anywhere else would have been inconvenient and wouldn't have saved him the time he wanted to save. It is definitely NOT the Longbourn of Jane Austen.
The Longborne Flying Club is fictional, as is the town or district of that name.