There are no specific heated elements in the cargo hold; it is kept at the same pressure as the rest of the cabin and is climate controlled (so things don't freeze over).
The only time there will be "extra" heat is if there is a live animal being transported on your flight, and then its constrained to the area where the animal is.
In short, you should not have issues with your food being spoiled.
Now, should you freeze the fried fish? If you are on a long flight, then I suggest you do; however I have yet to find any kind of fried food that tasted good after freezing (and then thawing it).
My suggestion is to store it in a thermal sealed box (like an ice-chest). This way it will retain its heat.
Cosmetics should be okay as they are only affected by extreme variances in temperature (ie, either it gets very very hot, or very very cold).
It does not matter the airline you travel with - as most major airliners have cargo holds that are pressure and environment controlled.
The risk you have is if your flight is delayed - you may end up with your luggage being left on the tarmac for a short period of time; where its exposed to the elements and may even get rained on; but you run that risk no matter what airline you pick.
Finally, I would suggest going over the food guidelines of the UK border agency to make sure you aren't bringing in any prohibited or restricted items. There are very few restrictions on EU - UK flights, the majority of restrictions are from external (non-EU) to UK flights.
