Lying to the air carrier to commit fraud is the worst suggestion of all. At least the other suggestions are helpful and can't hurt. If you say that there is a dead relative and you can't produce a death certificate, you then have notes in the PNR and a more reasoned approach won't work:

First, have you asked the carrier, in person, whether it might waive the change fee because you have no money at all? No idea of your carrier, but a city ticket office or, if necessary, the airport counter, would be best. Next is a call to the carrier. Don't make this overly-dramatic, stick to the facts, and ask for the waiver.

Second, while the Consulate is a last resort, this is a GBP 100 change fee, not an air ticket and to the extent that you will wind up making a local public assistance complaint and possibly being booted out, you may find that a consular officer intervenes with a request to the carrier for a waiver of the fee.

Third, you may not have relatives or friends locally, but the fee can be wired or more importantly paid, pretty much anywhere. These could be relatives or friends anywhere in the world who can come up collectively with the equivalent of GBP 100 pretty much anywhere. Without prying, is there really no one person or even a group of people who can come up with that sum for you somewhere in the world?

As to the "living at airport" issue, I doubt that it works, but it is also a bad idea because food will be very expensive and that will far outweigh whatever else you might save.