Answering in order asked:
Yes, you can use your Visa CREDIT card to get a cash advance from an ATM. Some banks may allow this too, but most simply point to their ATM when you ask. But it is expensive and should only be done as a last resort. And a cash advance is NEVER included in the 0% interest period! Look at your credit card agreement, it will state that only purchases are 0%. You will pay some fee for each and every cash advance, whether a flat amount or a percentage, and the interest charges start as soon as the money comes out of the ATM. Use your ATM Debit card instead. Check with your bank to find out what fees there are for getting cash from foreign ATMs.
No, you cannot exchange any cash at a bank anymore unless you have an account with them. A side effect of having ATMs everywhere, but banks just don't want the hassle of handling foreign cash. You may find an exchange booth at the ferry pier that will exchange Euro for Pounds if you have an excessive amount left, but the rate will be terribly bad.
Since you have a B&B to pay on arrival, in this case it makes sense to have some Euro in your pocket. But still plan on getting most of your cash from an ATM with your debit card.
Although not asked: don't forget to inform your credit card issuers and your debit card banks when and where you will be in Europe so they don't shut down your cards when they start seeing transactions from Europe. Also, get your credit card PINs. You may never need it for purchases while in Europe, but it is good to have the PIN if it is required.