Whilst not trying to disagree with the petition, and I've signed......
The reason the Lifeboat Service is VAT free is its a charity and the Lifeboat men and women are unpaid volunteers.
The Air Ambulance is a great service and funded a lot by charitable donations, but it ain't wholely charitable donations and the employees, paramedics and pilots, are paid. I'm not sure it would/could work like the lifeboat service.