The total number of full-time equivalent teachers that left the profession before retirement in each year since 2010 is published in Table 7b of the ‘School Workforce in England: November 2015’ statistical release: https://www.gov.uk/government/statistics/school-workforce-in-england-november-2015
The table in the link shows the number of teachers that were in service in a state funded school in England in one year but had left service by the following year. Some of these teachers could have moved to teach outside the state funded sector (e.g. in an independent school) or moved to the education sectors in another UK country. However, the exact numbers are not known.
Furthermore, a number of these teachers will return to the profession. Table 7b also shows that, for example, in 2015 over 14,000 ex teachers returned to work in a state funded school in England.
The number of teachers that left the profession before retirement to teach overseas is not known.