。
When she lived in London, she used to go to that theater often.
Literal
[topic-は] London [in-に] was-located time often that theater [to-に] went [used-to-ものだ].
~たものだ ('used to do') is the nostalgic past — a way of recalling habitual actions from the past with a tinge of fondness or wistfulness. It's distinct from plain ~た, which just states a past event. ころ ('around the time, when') is a softer alternative to 時 — it suggests a vaguer time frame ('back when she was in London'), often used for reminiscence. The verb いる appears here in its locational sense ('was in London'), which is interchangeable with 住んでいる for a stay but slightly less committal — useful for shorter periods.