I'd suggest you find a bank in your local country, and consider opening a Euro denominated bank account with them. Depending on your other banking needs, you might even be able to get this account for free. Otherwise, you'll likely have to pay a monthly fee, but you'll only want it for a month or two.
(Many banks offer accounts in other major world currencies, but not all. Ask your current bank, then failing that try a few others. As you say you're in the UK, try CitiBank - they charge fees unless you use them as your main account, but lots of people who travel a lot really like them)
Then, when you have the account open, pay the cash in. Finally, when abroad, use the cash card from that account to pay for things in Euros, or take cash out. When you're done with the trip, if the fees are too high, close the account.
In general though, your best bet for spending money abroad is detailed in this answer - get a cash card with no overseas fees and withdraw from an ATM. If you go somewhere a lot, it may make sense to open a bank account in that currency, to avoid exchange rate risks. Depending on costs and the countries involved, that might make sense to be a bank account opening in your home country in a different currency, or in a country in that currency (as an overseas / ex-pat account)