Seeking some advice on this one :

Global US-HQd corporation (think IBM-sized), with a long-standing Marriott corporate code, which grants corporate rates at Marriott hotels in the US.

In the UK, there's about a dozen facilities around the country, but the corporate code only works at one Marriott in the whole country, which is near one of the regional facilities. Code doesn't work anywhere else, not even in any of the properties in the London area.

Employees with newly-matched Marriott elite status from SPG want to stay at Marriott properties when they're travelling around the UK rather than, say, the usual Hilton.

What's the process of getting the code extended to be valid at Marriott properties closest to the regional facilities ? Is it simply a case of contacting the property, or does it have to be done centrally via the US or UK HQ ?