I can tell you for the UK first hand.
I bought around 4k worth of gear in Italy, crossed the border to Switzerland and picked up 200 prop 50ml. Left the next day and flew into Manchester with everything in 1 big bag (BIG bag).
Of course, wearing sunglasses (December), built like a brickshithouse and carrying this big bag had to attract attention which it did to the joy of the 2 custom blokes who first opened the bag. This was then extended to about 8 guys who unpacked the contents in front of everybody going through customs. For 20 minutes I had to wait for the 'specialist' who then made a note of how much of this, how much of that etc.
"Question 1" he said, "who is this for?"
"Myself" I said, "Correct" was his answer as though I was in some kinda quiz.
"and how long will this last you?"
"3 months" was my reply
"Correct again, obviously you know the law and we are wasting our time, thank you and good day".
They then packed my bag, folded the only T-shirt and sent me on my way.
Since then I have done the same trip 3 times, twiced check on the Swiss border and never had a problem.
Obviously the amount I carried was just enough for myself for a period of 3 months
If unsure, ask customs, I did !