Have just watched an old Men&Motors Youtube video, wherein a Traffic Police officer wrongly stated you cannot even overtake a car that moves over to its nearside on a wide road to let you pass, even if you don't cross the solid white line on your side.

I know that this particular officer is/was actually a very good 'un, but it just shows how much open to personal interpretation / confusion this issue causes.
Nowhere, I mean nowhere, can I find a definition of 'stationary' within the Highway code or RTA1988, but the English dictionary does state this.....
Meaning of stationary in English
stationary
adjective
UK
Add to word list
not
moving, or not
changing:
a stationary
car/
train
The traffic got slower and slower until it was stationary.
And I think that last part for me says it all.
I am and will continue to be very careful on my travels, but if the occasion arose whereby I was going to sit behind a mile or three of cages this summer, I'd seriously consider, if safe to do so, overtaking those stationary vehicles by crossing any solid white line on my side........... IF SAFE TO DO SO