It is clearly a Defiant fighter, note the large underbody radiator tub and the two piece landing gear doors.
The odd thing as you do point out is the normal glazed four gun turret appears to be replaced by a single weapon turret. Or perhaps that is not a gun but some other device. It might have been a trial armoured turret.
Send the photo to these chaps, they might be able to help:
http://www.boultonpaul.com/
Dennis