AMEN. Scummy rotters indeed.
First of all, so sorry for your loss, OP. And YANBU, of course, don't listen to all the 'celebration of life', 'but it's comfy', 'be grateful they showed up' idiots. You're absolutely right; dress respectfully or don't bother attending. 'Would you rather they came in joggers or not at all' - NOT AT ALL, obviously; who needs chavs at a funeral of a loved one.
I'm European, in my 30s, and no, no one specifically gives us lectures on what to wear at funerals. You know why? BECAUSE IT'S OBVIOUS, SHERLOCK! My 13-year-old teen knows what to wear, because she has brains and eyes, never mind actual adults. Wear black, formal clothing, cover up, no tits&bits, no chav in the shops attire; I mean, does it really need spelling out?
It changed a bit in recent years; now closest family are suited and booted, all black (regardless of a 'massive heatwave of 31 degrees', are you having a laugh here with such a 'heatwave'). Others can wear different colours, but all dark, no prints. No shorts, joggers, flip-flops, strappy tops and shit like that, jesus.
We had one dragged-up skank showing up to my grandad's funeral. He was a well-known man, and the funeral was large, all people in black pretty much. She rocked up in a red maxi dress, with her daughter in a crop top. They were asked to leave as soon as they stepped inside.