I don’t know if FlixBus have ‘standards’ for their operators, but I took the overnight bus from Leeds to London last night, and the seats were really close to each other. It was a newish Yutong, but unfortunately I didn’t note the operator. It used to be Whippet, and although their shortcomings have been discussed on here frequently, the seats and legroom were comfortable.
I’m only five foot five and my knees were nearly touching the seat in front. There was very little room to push my rucksack under the seat in front, and it wouldn’t have fitted in the overhead rack.
By contrast, I got a Slovakian FlixBus onward to Brussels: there was at least six inches between my knees and the seat in front, under which my rucksack easily fitted. It was a Setra.
I did notice a few Yutongs at Victoria coach station bearing the FlixBus livery. I really hope this is not the new standard - which I assume means squeezing an extra row of seats in. I have had nothing but positive experiences with them before.