Trick or Treat

Apparently originated in Canada in the 1920s, who copied it from "(dis)guising" from Scotland/Ireland.

Guising / Souling

involved kids singing at a door around October, or November.

"Ma feet's cauld, my shune's thin;
Please my cakes an' let me rin."

After which they'd get cakes and lemonade

English people claimed it was a weird invention from the US when it came over in the 1980s. Scots and Irish though it was just a weird resurgence of Guising.

Pumpkin

Not native to the UK, we used to put candles in turnips. It migrated to the US and they changed it to a pumpkin and we took it back that way

Guy Fawkes

Kids used to wheel around a straw effigy and ask "penny for the guy" but apparently (according to BBC and Wikipedia) it's a new invention (1910) invented by Big Fireworks to sell fireworks on 5th November to kids as "Fireworks Night".