It's refreshing to see a ship related question

It certainly looks like the 4 inch gun on a flower class. Those things in the background may be some kind of smoke generator. Merchant Ships also had a gun on the stern so that is a possibility but if I had to guess I'd say it was a warship.

I can't answer your question but may be able to point you in the direction of people that can.

There is a Flower Class Corvette Message Board that may be of help. It doesn't seem very active at the moment but may be worth a try.

https://www.tapatalk.com/groups/theflowerclasscorvetteforums/#.UptEjaUR8pE

There is also a "Flower Class Corvette Association" Group on Facebook which is pretty active and is probably the place that you are most likely to get an answer.

Good luck.

Terry