Hi omglolmax,
If you pay for something in Italy using your UK bank issues debit card, then the conversion is done at your UK bank when the transaction is processed. You'll pay the UK bank you are with rate for foreign transactions. The 'live' actually happening on the stock exchange currency rate you won't be paying.
It's the same if you use your debit card on one of Amazon's local country websites. You'll pay the exchange rate your UK bank has set on that day once the transaction occurs on your account.
The difference in rates for purchasing things on holiday or for online shopping means that even if you found a rate that's 'excellent' it'll perhaps make a difference of a few Euro's or Pounds. This is because most purchases like this will be up to perhaps several hundred pounds for the most part, so a difference of one rate to another being say £10-30 or so most people won't bother trying to find the best rate.
When it comes to larger amounts, thousands - hundreds of thousands usually, then it's always best to look for a specific Currency Exchange specialist who get and then offer better deals than the high street banks.
Disclaimer - Without going into too much intricacy, when you swipe your card in another country (keeping it European for now) then between that swipe and your account, there's perhaps 80 or more financial institutions that could be 'carrying' your data. This all can happen extremely quickly, but the reason you see it on your account a day or two later is down to the processing schedule as well as how quick the data is handled by various servers. The cynical will say it's deliberate as more 'interest' can be charged in the time frame (which to an extent is true!) however when you consider that in 2008, one of the UK's main financial processing data centres when the Consultancy I worked at had a presence, then the servers there were handling around 2 million financial transactions every second, I can only guess this has increased fairly substantially over the last few years. I think the 'slowest' part of the whole entire process is indeed your UK bank, and it taking it's time to 'transfer' the amount onto your account so they scrape that little bit of interest !!
Cheers, DtM! West London & Slough UK!