Is there a rubber gasket around the frame? Can’t quite tell from the pics. First option may be to replace the rubber gasket - very simple and easy to do.
The rubber compresses over time so changing it over can make the fit much more airtight.
You can get it from Toolstation in a a couple of colours. It’s by Stormguard - https://www.toolstation.com/stormgua...al-seal/p57199
I used it for some windows last year and it made a big difference - the windows are now also much harder to close as the rubber hasn't compressed down.