Have you tried changing the TM target language to plain-vanilla EN?
Or, duplicate your TM into an EN-US TM, and then use both of them for each job?
Then again, doing so will make spell-checking unreliable.
This was the advice from Wordfast:
"In your case one solution would be to export the TM as TMX and then Find/Replace EN-GB locale with EN locale, then import the TM back to WF as EN locale TM inside EN-GB and EN-US target language projects. This way your TM is always EN and can be used in any country variant locale project like EN-US/EN-GB. This way your spellchecker is fine for the project."
I did that (export the TM to tmx format, open with text editor, replace EN-GB and EN-US with EN, save) and tried to import this back into Wordfast. At that point I got an error message about the header of the TM but I was able to resolve that using the WfClassic data editor (as per Wordfast Help page on error messages). I was then able to import this "plain vanilla" TM. The only other thing I had to make sure of was the project as such didn't already specify UK or US English as target but was just set for English.
Hopefully this should work now; I'll update on here if not.