The FCO have published the following advise
On 29 October the new airport in Istanbul is due to open with flights transferring from the existing Ataturk airport between 29 and 31 October. During the transition period, flights are expected to operate simultaneously from both airports. You should allow sufficient time forcheck in, transfers to onward destinations and for potential transfer between the two airports. If you have any questions regarding your flight you should contact your airline for information.
https://www.gov.uk/foreign-travel-advice/turkey