Hi Ed,
5366 - 5366.5 is within the UK bandlet at 5362 - 5374.5 and the RSGB bandplan for that segment says:
“All modes. USB 5363kHz, 5371.5kHz. WRC-15 band to 5366.5kHz. Weak signal 5366.0 – 5366.5kHz”
So UK operators could use 5366kHz for FT8, but with the upper edge of the WRC15 band being at 5366.5 continental operators wanting to contact them would be restricted to offsets up to 450Hz. And TBH I’m not sure that I’d class FT8 as a weak signal mode. Also don’t forget that although any individual signal is no more than 25Hz wide, the mode itself can occupy a band up to about 2600Hz above the carrier frequency.
The “de facto” carrier frequency for 60m FT8 is 5357kHz, I’ve not found any operation away from there so we in the UK are stuck with being careful 
73 de Paul G4MD