I was looking online for what countries the British Museum has returned items to. However I cannot find any sources that give a comprehensive list. Where can I find this information?
Partial answer, but perhaps a partial answer will help someone to create a better answer.
First, note that it is illegal under British law for the British Museum to repatriate artifacts.
The British Museum has long been barred from repatriation under a 60-year-old law. But public pressure is mounting for a change. Observer.com
Under the act, the British Museum’s board of trustees is barred from returning any object in the collection unless it is a duplicate, physically damaged or “unfit to be retained the collection” and no longer of public interest, according to the museum’s deaccessioning policy. Other national museums in the U.K., such as London’s Victoria and Albert Museum, are also prevented from repatriating works under the National Heritage Act of 1983. Ibid
and (hat tip to @ccprog), for the human remains, the museum maintains its own public documentation outlining law and policy.
That said, apparently there are some examples.
It says it has received two requests for repatriation since the Human Tissue Act 2004 came into force in 2005: the first in July that year for two cremation ash bundles from the Tasmanian Aboriginal Centre and another in May 2011 for two skulls from a Torres Strait Islander organisation, in both instances in cooperation with the Australian government.
Following a review, the ash was ultimately returned but the skulls were retained after the trustees ruled there was insufficient evidence to support the claim on them. Independent.co.uk
A British museum (not the British Museum) also also repatriated some Nigerian artifacts.
A British museum will return 72 artifacts to the Nigerian government that were forcibly taken over a hundred years ago. ABC.go.com