Key problem is you can’t get medical thermometers at the moment here. But if you have a reference one and a thermocouple you can get an indicative reading in the short term at least good to 0.5oC or so
I've got two, the latter a couple of weeks ago when I realised I couldn't change the batteries in my first.
I've also got a mercury thermometer, but I'm not sure I could read it correctly with a fever. Somehow ringing 111/999 with "I can't read my thermometer so I need to be admitted" isn't likely to have the desired effect.
The other technique is to hold your breath while putting a point-and-shoot non-contact "gun" in your mouth, and waving it around to get the max temperature. Captures the same temperature to within, say, 0.2C