Conversation so far:
Are there any other interesting aspects about Mary Shelley's article?
On their return to England in September, Mary Shelley and Percy Shelley moved—with Claire Clairmont, who took lodgings nearby—to Bath, where they hoped to keep Claire's pregnancy secret.
Who are bath and marlow in relation to Mary Shelley?
On their return to England in September, Mary Shelley and Percy Shelley moved to Bath. In March, the Shelleys moved to Albion House at Marlow, Buckinghamshire.
Where was bath located?
Bath is the largest city in the county of Somerset, England.
Did Mary Shelley and her family stay in Bath long?
Did the Shelleys stay in Marlow with Claire Clairmont for a while?
The threat of a debtor's prison, combined with fears of losing custody of their children, contributed to the couple's decision to leave England on 12 March 1818.
Why did Mary Shelley fear losing custody?
Harriet's family obstructed Percy Shelley's efforts—fully supported by Mary Godwin—to assume custody of his two children by Harriet.

Question: Why were they in debt?
Self-contained rewrite: Why was Mary Shelley's family in debt?