Like everyone says easiest is to use a debit card on a current account with bank that charges no fees and uses direct rate: nationwide, virgin, chase, starling, monzo, first direct, probably more. Easy enough to open one specially if necessary.
Currensea is pretty good if you bank elsewhere and can’t be bothered opening a new account. It converts and then direct debits (a few days later) in GBP from your existing current account. No fees but the rate is a bit worse than those above. Better than “taking the hit” though and easy to set up.
HSBC Global Money is similar rates to Currensea (i.e. a bit worse than the bank accounts above) but you have to move the money into the account yourself. But that’s a faff – if you do that, why no just get a full bank account elsewhere for the better rate? Wise works in similar fashion (but with better rate, like those banks, I think).
Revolut is a weird cult, not a bank, and is more expensive than the banks above. (I do actually use it for the kids’ pocket money. Not for travel though. And people recommend it for *everything*, often claiming its better than an actual bank, for no good reason.)
Clarity is good, best credit card for this, but no better than the debit cards above for purchases and you’ll pay interest on cash withdrawals.