Detective in me thinks this may be his gf's face:
https://www.instagram.com/p/Cc2cszcNQVf/
I don't know how you embed insta. But anyway, this is from April. You guys posted it on page 7.
Since we know he doesn't draw his gf's face when he draws her nudes, perhaps he draws her face separately. So nudie from June:
https://www.instagram.com/p/CfBkLxxNCqF/
plus portrait from April = gf?
I see that he went to Glastonbury with his long-time friend Victoria (she's the woman in a red jacket and shades) and her husband who took the photos that you posted on the previous page. Hard to tell if there was anyone else. But anway, the woman in his drawing isn't his mother, isn't Victoria, doesn't look like his sister Kine either. Also, since he drew her face from different angles, it cannot be off a photo. So unless he went to an actual drawing class with a model, it should be someone from real life.