There are two schools of thought about getting money here before you go, and zillions of posts restating the two positions over and over and over again.
1) Get $100-200 worth of the European currency here, before you go. This way, you don't have to hassle with it upon arrival. What if the airport ATM isn't working, or there's a long line?
2) Wait until your arrival airport to get local currency. You'll get a better rate, you don't have to carry foreign currency with you, and these days, you can usually charge most things, so you may not need cash right way anyway.
Your choice.
You definitely don't need to get both pounds and euros before you go. If you do want to have some cash on hand upon arrival, just get the first currency you'll need. And you don't need more than $100-200 worth of foreign currency; once there, you'll be using ATM's for cash, and credit cards to charge things.
If you know anyone who's been traveling and may have some leftover pounds or euros, see if you can buy these from them. This way, you get the best rate of all.
For more money tips, read all the links on Rick's money page, and you'll be an expert: https://www.ricksteves.com/travel-tips/money