To accept credit cards, small companies have to take a contract with a "payment provider". These make websites that look like a bank. So when you order from such a small company, they will redirect you to the payment provider who will ask for your credit card and CVC (that's what it's called here).
Bigger companies, apparently Farnell included, can work with the creditcard company directly. So they will have to ask for the verification number themselves.
If you don't dare to give Farnell your creditcard number and/or verfication code, then Farnell will not be able to charge your credit card. And they will not send a Raspberry pi to you. Simple.
Actually giving your CVC to one of those credit card processing companies is quite dangerous compared to Farnell. If Farnell sticks to the rules, they will throw away your credit card details as soon as possible after finishing the transaction. So if a hacker hacks into Farnell just a few credit card numbers will be there (maybe hundreds, maybe thousands).
But if the hacker gets into the credit card processing company, they will have a MUCH larger loot. Moreover, even IF nothing gets stored, they could tap the live "feed" of credit cards and steal that!
My advice: don't worry, be happy.
Check out our raspberry pi addons: https://www.bitwizard.nl/shop/