The British Museum Act forbids the British Museum from de-accessioning artefacts from its collection, unless they are duplicates of other items or damaged to the extent of being worthless. Attempts to legally circumvent it  have been unsuccessful . This fact is regularly used as a wall by the British Museum when anyone asks to discuss the reunification of the Elgin Marbles – with the statement that even if they wanted to return them they couldn’t.
A solution to this has been put forward in the past by the Greek Government, suggesting that the sculptures could instead be located in the New Acropolis Museum on a long term loan – an arrangement that is supported by many in Britain . The British Museum has in the past avoided serious discussion of this, by stating that the concept of long term loans is oxymoronic – suggesting that a loan for a long duration is no longer a loan & essentially constitutes ownership, making it impossible.
A few weeks ago, I covered one artefact – currently in the British Museum  on long term loan. It appears though that in their collection are many other similar cases, such as the chalice from Lacock detailed in the article below, which has been on loan to the British Museum since the 1960s. Clearly long term loans are a lot easier to contemplate when you are the recipient rather than the owner – but whatever point of view one takes on that, it is clear that long terms loans are very definitely possible.
Daily Telegraph 
£2 million communion chalice could save church roof
A church appealing to raise money for a new roof has had its prayers answered after one of its silver communion chalices was valued at £2 million
Published: 7:00AM BST 28 Sep 2009
The medieval cup, which stands just a few inches high, was described by experts as one of the best-preserved specimens of its kind anywhere in the world.
It has been used by countless generations of worshippers at St Cyriac’s Church in the village of Lacock, Wilts, since the 1400s. But until now the chalice – on loan to the British Museum since the 1960s – has never been accurately valued.
Officials at the 15th-Century church – which held the marriage of Laura Parker Bowles, the daughter of the Duchess of Cornwall – are now considering selling it to fund a £350,000 roof restoration project.
The Venerable Alan Hawker, Archdeacon of Malmesbury, said the remaining cash would be put into a trust fund and generate £50,000-a-year interest for the village.
This week, more than 50 local residents attended a public meeting about the proposed sale.
Many were concerned about losing a part of the village’s heritage, while others questioned whether selling it in a recession would decrease its value.
The chalice was given to the church by an unknown benefactor around 400 years ago, and is said to date back to 1400.
It has been on loan to the British museum since 1963, where it is on display in the Edward VII gallery.
Now the museum wants to buy it permanently. Its valuers priced it at £2 million – £200,000 more than it is currently insured for.
But selling Church-owned silver is not easy. After a period of consultation with villagers, Church authorities must apply for a ‘faculty’ – a kind of diocesan planning application.
St Cyriac’s vicar, the Reverend Sally Wheeler, said the proposals were still at an early stage.
She said: “We will no go and look at people’s letters and opinions and decide what to do next.”
The church hit the headlines in May 2006 when Laura Parker Bowles and Calvin Klein model Harry Lopes were married there.