Thanks for the additional information and no worries about the delay:)
Gum color is normal which is good as is the fact that he's urinating (so we can eliminate a urinary blockage from the list) and no abdominal pain.
There could be several possible explanations for his symptoms but since he's a little brighter today, that's a good sign:
1. Fever. Unfortuantely, you can't tell much about the core temperature of a cat without actually taking their temperature. If you manage to get your hands on a rectal thermometer, you could take his temperature although this is often very hard to do in a cat. Normal is between 100 and 102.5.
Of course, then we'd have to wonder why the temperature is elevated. Explanations for this could be: a bacterial infection, a virus (such as leukemia, AIDS, FIP), or toxoplasmosis.
There is a condition in cats that is called Fever of Unknown Origin--which is exactly what it sounds like in that we can't find an explanation for the elevated temperature. Treatment is primarily supportive which consists of fluids and drugs to reduce the fever. Most cats recover very quickly once the fever is reduced.
2. Pancreatitis can happen very suddenly; appetites can be affected and they can become lethargic. Approximately 30% of cats with this condition will vomit. We don't have a good explanation for why this happens, either.
We do have a test for this (spec fPL) although I've come to doubt its reliability in some cases; treatment consists of fluids and pain medication (we believe the pain is why they stop eating).
3. Other systemic diseases such as kidney disease, liver disease, anemia, problems with the heart, etc are all possible but usually there's other symptoms.
4. I've seen a few cats just have "off" days but they're usually showing improvement after 2-3 days so he's pushing the limits if this might be the problem.
Unfortuantely, it's not considered advisable or safe to give cats many over the counter drugs since cats are so sensitive to many of them and they could cause more harm. But, if he's nauseous, he could be given 1/4th of a Famotidine twice a day which might help.This drug may be difficult to find in the UK so Omeprazole (Prilosec) can also be given at a dose of1/2th of a 10 mg tablet once a day
Maintaining hydration with water or Pedialyte is important, especially if there’s a fever. If he's not drinking very much, you can dribble this into the side of the mouth with an eye dropper or syringe every 30-60 minutes. You can also can mix canned food with a lot of water or tuna juice and see if you can get him to "drink" his meals.
Since he appears to be a little perkier today, you could continue to monitor him; however, if he's not continuing in this positive direction, then a vet visit might be prudent.
I hope this helps and that he's feeling better soon. Deb