Given the information that you have given , the births should registered with the British Consulate.
If you care to venture, the GRO holds a microfilm set, as with nowadays most major libraries and on the Find My Past site which may (almost certainly ) give the reference number for the birth certificate as he was a foreign national.
As for other information, I can suggest .... with some possible success, that the family may be found on some shipping records more so than immigration/passport control documents....... I would enter "Freeman , passenger lists [or words to effect] in google or hope that some kind soul in RC will do so to see if there are any surviving records.....
Good luck with that, and I would also put a solid post up on War Forum to see if a person worldwide knows more about military machinery contracts made in Turkey...... Given the period I also believe "supply" contracts were being given to India..?
marty
