Your UK bank will charge you about 2.5% above the official "mid-rate" exchange rate to use your debit card. Check their website for exact numbers.
The main thing to avoid is if there is a minimum charge for a transaction, as it's annoying to pay €3.50 for two coffees and pay £1 extra in charges. I have a NatWest card and there is no minimum charge.
You can pay less than that 2.5% if you use a card like Revolut or Wise (about 0.4%), but unless you bank with them you will find yourself having to transfer money to them from your main online banking to keep it topped up. Also note that there are quite low limits on how much cash you can take out at a machine with these cards.
If the card terminal asks if you want to pay in Euros or pounds, always choose Euros. The terminal will add on a lot more than 2.5% to convert.
Using your card, even with the 2.5%, can work out cheaper than using cash, as you pay a decent percentage to convert the cash, and then again when you convert any unused money back.
Under no circumstances change money at a hotel, or an airport bureau de change, or use an airport cash machine to withdraw currency. The rates they give are horrible. Also avoid cash machines in the street at your resort. Use ones that are attached to a physical bank branch. There may be a charge of about €2.50 for each withdrawal, but the exchange rate will be OK. The ones at airports and in the resort give horrible rates.
One of the best places to get foreign currency in the UK can be your local high street pawn shop (seriously). But wherever you are looking to change money, ask "Here is £1000. Exactly how many Euros will I leave with?" and get a chit with that on before you hand over the cash. Too often there is an unmentioned % commission or £10 "handling fee" that you only find out about as you leave.