Hi rome:

Being overweight does not have so much to do with counting calories as it does with overeating. For 5:2, the book’s advice is to eat 500 cal. or fewer on two diet days each week, and to eat ‘normally’ on the remaining five. The question becomes ‘What does normally mean?’

Some come to 5:2 and, assuming they are doing their diet days correctly, still find they do not lose, and may even gain, weight. That is because they are overeating on their non-diet days. The only way for them to correct that is to figure out their TDEE and count calories for a week or two to see how much they have been eating in relation to their TDEE. On discovering how much they are overeating, some give up the diet because they want to continue eating that much food. Others go about reducing their eating on their non-diet days until they start losing weight at an acceptable rate.

At the start, you don’t have to count calories on your non-diet days if you don’t want to. Do a month or two of correct two days a week diet days and see how much weight, if any, you have lost after that time. Then adjust. If you are not losing as fast as you like, you still don’t have to count calories – just make sure you start eating less on your non-diet days.

The less you eat, the more you lose.

Here are some tips: https://thefastdiet.co.uk/forums/topic/the-basics-for-newbies-your-questions-answered/

Good Luck!