Given that Julie had recently abandoned her baby two or three episodes prior?

Ann Romano's move to London was primarily driven by a significant career opportunity and was written to provide a narrative exit for her character as the show was ending. However, you are right to question the timing from a character perspective; the decision came long after Julie had moved away and started her own life, not immediately after the baby was abandoned.

Character & Storyline Separation
The events you are referencing occurred in different seasons and were handled independently by the show's writers:

Julie's Disappearance and Baby Abandonment
Julie left the show several times due to actress Mackenzie Phillips' personal struggles. The plotline where she leaves her baby, "Little Annie," with Ann and then disappears was a major, dramatic arc used to write the character out of the show for a period. This happened earlier in the show's later seasons.

Ann's Move to London
This happened in the penultimate episode of the entire series, at the end of Season 9 (1984). By this point, both daughters were grown, married, and had their own lives. Julie was off-screen, and Barbara and her husband Mark were living in a home with Ann's mother.

The Real-World Reason
Ultimately, the decision was less about the in-show drama and more about real-world production changes:

Actress Departure: Bonnie Franklin (Ann Romano) informed producers she was leaving the show after the ninth season.

Storyline Conclusion: The writers needed a way to conclude Ann's story that honored her character's arc as a woman who prioritized her career and independence. Moving to London for a prestigious job in advertising (with her new husband, Sam Royer, in tow) provided that closure, showing she achieved the self-sufficiency she always wanted.

Spin-off Attempts: The final episodes were also designed as "backdoor pilots" to launch spin-off shows for the remaining cast members, such as Schneider moving to Florida.

So while it felt sudden from a viewer's perspective, within the show's final timeline, Ann wasn't leaving a family in crisis. She was leaving an empty nest to start a new, well-earned chapter of her life