UK want to avoid court proceedings concerning the sovereignty of the Chagos Island, which may cause the suspension of the air base, and so have agreed to hand the islands back to Mauritius, and then to lease the airfield back for at least 100 years.
Trump says UK should keep the islands but it's highly likely this contravenes international law. It's strategically an extremely important military/air base used by UK/USA.
Many Chagos people consider themselves a nation and don't want to be part of Mauritius. UK has offered them British citizenship and many have made their way here, to Crawley. They have to fund their own travel. Many Chagos people hope one day to be able to return to independent Chagos Islands and there is a British charitable organisation supporting this, that charity has help 4 Chagos islanders to return there recently.
Frankly it's a right mess but if we don't come to some agreement it will all get tied up in court cases (possibly covertly funded by China), and the right to use the air base will be suspended.
Starmer wants to compromise so the base is secure.
Trump wants to do what Trump wants to do.