tbh, I think the fault is more with the OP (and possibly a lack of experience/forethought that a car with indicators on, crawling next to a row of cars, might want to reverse-park into the space). (dare I ask whether this was a last minute swerve/overtake to avoid rear-ending her, if the OP wasn't expecting her to start reverse-parking towards him?!? - or was it stationary traffic, she's just crawled past a parking space, indicated, stopped and the OP has started to overtake while she was putting it into reverse - again, OP rushing and not considering other traffic)
The parking car indicated and did everything correct (up until the impact) according the the OP. Some people are suggesting she could have stopped when she saw the OP overtake, but how do we know she didn't? It sounds like she indicated, waited for a gap and started the manoeuvre, thus the car is swinging out wider, the OP starts to overtake (which probably suggests he didn't leave enough space for her to park or was a last minute swerve to avoid rear-ending, as who the flip overtakes a parking car...), whether she stops or not, the OP is going to hit her.
While I think the insurance will probably go 50/50. I guess it could be possibly swayed in her favour if she has a dashcam to show she stopped prior to the impact, or the OP driving like a prat, or in the OP's favour if he has a dashcam showing she started to reverse-park while he was still approaching her and she didn't wait for a suitable time/gap to park... but I think the blame is more on the OP, and evidently not giving enough space while overtaking.
She was parking, and it was the OP's decision to overtake the caused the accident. without him doing that manoeuvre there wouldn't have been the accident, so I think she's got an argument against the OP...