Hi there,
If you wouldn't mind just trying two more steps for me, I think we can sort this out.
If you attempt to uninstall the default Asobo airports by deleting the folder; they'll be reinstalled on the next sim launch. To uninstall them and not have them reinstalled; you need to do the following steps:
1 - Go to your Profile at the MSFS home screen.
2 - Select Content Manager.
3 - Search "EGLL" in the search bar.
This will then either bring up the airport directly, at which point you can click it, the click delete in the bottom right corner.
If it instead shows you the premium deluxe upgrade (which EGLL was included with); you need to click that item, then an icon with 3 dots and 3 lines should appear next to it. Clicking that will list everything included with that upgrade; including EGLL. You can then delete it from there following the step above.
Finally, for good measure; clear your rolling cache. Options > General > Data > Delete Rolling Cache File
Please do try these at your earliest convenience and let me know the results