uPVC door jams when trying to open it from the inside...

[mcprinter]

That could just be your door handle. It looks well used
Often they have spring cassettes in the them to re-centre them.

This type of thing

https://www.amazon.co.uk/uPVC-Replacement-Handle-Springs-Cassette/dp/B07BWJ7KT8

Replacement handles are not too bad if you wanted to replace £20ish
Replacing the thumb turn would require a new lock I think £20ish
Looks good enough from a distance. Just the close up shows the marks

To fix your problem, I would personally replace the whole mechanism rather than faff with rivets on just the centre.

 

[Harry Bloomfield]

I notice that when the door is closed and locked, the handle sags downwards rather than being horizontal. Maybe that also implicates the gearbox.

That could imply wear in the square socket, within the handles, and/or the square shaft which goes right through the door and the mechanism. Try removing the handles and checking those for wear.

 

[diy_fun_uk]

Unless you're on a budget, just replace the whole lot and be done with it save the hassle of is it part a, b, c or d that's to blame.

 

[crank39]

Yes it does