Short answer, you're doing nothing wrong, and I'd do it just as you are.
That's correct.
But it's also incorrect.
The only place it is not possible for a road user to cause an obstruction is in a marked parking bay.
Blocking the traffic waiting to make a turn is an obstruction. If it's taking an undue amount of time to make the turn the obstruction becomes unreasonable and starts to cross the threshold into becoming an offence. If you can't make the manoeuvre in a reasonable period of time (and
reasonable takes in many factors and may ultimately be for a court to decide) then any road user should move on.
You can be knocked off for it, though I have never personally done so. I had a colleague who came across a car doing this and it was causing horrendous tailbacks and snarling up a second junction further back. He instructed the driver to move, and the driver refused - itself an offence for a driver to fail to obey an instruction given by a constable in uniform - so he got an FPN for unnecessary obstruction. The driver contested it in court, lost, and as I recall the case made the papers (the Telegraph, as I recall).
No matter the vehicle, if you can't complete the manoeuvre in a reasonable time them move along or pull over.