Bank of America allows you to order most foreign currency online and pick up at a Bank of America branch nearest to you. The rates are not going to be as good as you'd get at ATM's in London/Europe (depending on how the dollar performs between now and when you leave).

http://www.bankofamerica.com/foreigncurrency/

I'm not sure what you mean be being too "risky" to take your Visa/ATM to Europe. Many if not most ATM cards these days are linked to the VISA network. We've traveled all over the world with ours with no problems. If you're worried about losing your card and having someone clear out your bank account, then you can always move the bulk of your checking account money to an account that's not linked to your ATM/Visa card while you're on vacation then move it back when you get home. Your ATM/Visa card should have the same fraud protection as any other bank/credit card.

Have a great trip.