The above is good advice.
There are also two good Travelex products.The first is
Cash Passport.
Basically you get given a card which you have to pre-load with the main currencies that you want such as € or $.For example I got one a while back and whacked on some €s when the rate was 1.41 to the £.I'll do the same soon if the $ gets better.
The only downside is card must be used annually or they charge you,but not a problem for most people.The good thing is your money is protected if you lose the card and there are no foreign transaction fees which you normally get if you use your credit card abroad.
The second and newest card is the
Supercard which was launched in the last few days.
I applied for mine today but am on a waitlist as they have been inundated.It's similar to the Cash Passport but you don't have to preload it.You simply download the app to your iphone or android then apply through the app.When you get sent the card,you follow the instructions which will then link the Supercard to a credit card of your choice.
The benefit is you then use the Supercard just like your credit card but there are no foreign currency transaction fees,and unlike the Cash Passport you don't have to preload it as it just comes off your linked credit card account.The downside is you can't take advantage of favourable exchange rates as you can with the Cash Passport,as you will be charged the appropriate Travelex exchange rate for the currency in use on that day of sale.
The Supercard is especially useful for ATM withdrawals abroad as there will be no fees,though the exchange rate won't be quite as good as if you took the above advice of using Tesco or Sainsbury on pre-order.But you won't have to walk around with a wad of cash all the time.