McColgan is doubling at the World's. I don't think she has enough time to get in 2:18/19 shape in London. Maybe next year, after a second block of dedicated marathon training. Just looking at her fitness this year - 30:19 over 10k, and 66 for HM - I think she'll go for 2:20-21. I hope it's a very good day for her and she sneaks under 2:20 but anything under 2:23 has to be considered a great debut.
Another to watch is Mary Ngugi. She's quietly been stacking great results and her 2:21 PB is due for a revision. I think she could be added to he list of sub-2:19 women after this race