Retirements or loss of medical must be a large driver of demand in the U.K. A lot of the instructors are becoming 'sim only' simply due to loss of medical as a lot of them are more 'senior' in life. Also, the company is expanding all the time.
Overseas such as in the US or NZ, it's mostly because the instructors use the company to build hours then move off to the regionals as FOs.