Although Eurostar owns the depot, under UK and EU-derived rail legislation, key railway infrastructure must be made available on a fair and non-discriminatory basis. This includes maintenance depots if they are considered essential facilities. As Temple Mills is currently the only British railway depot accessible to (high-speed) trains with the larger continental European loading gauge, it's classed as essential for international rail services, so the rail regulator can issue an order to Eurostar to provide shared access.
Writing in The Times, Virgin founder Sir Richard Branson said his aim was to inject competition and improve the passenger experience: "For too long, passengers have had no choice and even less joy. We're not here to copy. We're here to raise standards, spark innovation and give people a better way to travel."
