This would seem like a no-brainer to me, as the concierge/guest services in Europe Marriotts are pretty phenomenal (there is no question in my mind that the Concierge at London County Hall would absolutely do this for a guest and I can't imagine that Grosvenor House (JW, right?) would be any different. (Don't you have to be fitted though? Do they have a different sizing schematic?) Anyway, call the reservations line and ask for the email address to the London property, them email them with your concierge request.
This is an excellent question to ask the forum, doctordave, and welcome to Insiders!
Have you tried contacting the property to verify? I'd be happy to retrieve contact information if you'd like.
Additionally, don't hesitate to reach out to me (joshm), CariJ, or lindseyh if you need any support while exploring the page or making any travel plans. Aside from the wonderful community that makes up this forum (They're always helpful), we make up the moderation team for the page!
Thanks for posting.