Subscribe to the Single App Illustrator plan as a completely separate membership under the same Adobe ID.
You'll be billed twice per month by Adobe: $10 for PS+LR and $20 for Illustrator.
Thank you John! Just did it.
Just to add my 2 thoughts.
I got in contact with a representative on the online Adobe chat, and they offered me a discounted £16 full app package for 1 year.
I'll probably do the same when this is due to renue next year, and hopefully get a similar deal.
£9 for photography is fine, but an additional £20 for 1 app seems a bit excessive.