Do you have status with CX? IIRC CX doesn’t do the credit card check for DMs.

While things may well have changed, I’d recommend that the OP contact CX beforehand- if nothing else to document the agreed solution (old card/letter from Amex) via a note in the booking. Admittedly not of late, but both the CX & SQ forums have had a fair share of posts from infrequent to CX/SQ travellers getting hassled for the credit card check.

An agent @ LGW should be able to sort this as well- I have done something similar at SIN for a booking that I had made for someone else- but I don’t have a clue about LGW CX hours/own personnel/ground agent ability etc.