Exchange Dollars into Pounds

I can't speak to that specific exchange rate, but generally I find the best rates are either (a) at your bank before you leave home; or (b) from a bank ATM (not a private ATM) in the country.

Get £s from an ATM in London

Even for small purchases, cards are accepted

Resist the urge to tip

I understand the urge to not spend unwisely, but %-wise whatever you pay at an ATM at Heathrow for X pounds to have in your wallet (that you will carry most of back and forth a number of times) is small compared to the cost your first meal in London (or Paris, or Rome). And don't ask about wine prices.

do you have a neighbourhood Facebook (or alternative) group you could ask people for odd notes and especially coins they may have brought home from trips? The £1 and £2 coins - and the odd shaped 50p - soon add up to quite a value but a lot of people treat them like shrapnel. But don't buy any paper notes, Bank of England notes are plastic (and don't accept £50 notes because traders don't really like them). A rate of $1.38 would be fair, the coins especially are hard to get rid of once taken to another country so you could be doing them a favour.

Within London most Post Offices (not banks) will change cash, ATMs should be signed as "free" - meaning they don't levy a service charge, but your card issuer might.

When are you going to use cash? You can use card for small purchases, market stalls, buskers, churches, pretty much everywhere. I travel to the UK 4-5 times a year and have had the same two £20 in my wallet for the past few years.

Tipping is not common and not normally expected. Most dining places have a service charge (again, payable by card).

If you really, really want cash, just withdraw from a cashpoint (ATM).

If by chance you are a Bank of America customer, they have foreign partners in a number of countries. In the UK, it's Barclays Bank. You can use their ATMs with no withdrawal fee from BoA or the partner bank. But there is a 3% currency exchange fee, so $1.39 per GBP if my math is right. If you aren't a BoA customer, check to see if your bank has any international affiliations.

The idea to try to find people who want to get rid of their foreign cash is a good one, because ATMs are going to give you large bills, and what you're really likely to need is smaller change (50p for the loo, sometimes!).

On the other hand, you're really not likely to need much cash, so you could change $50 or so in MD and don't worry about the extra few dollars the lousy exchange rate costs.

I've spent a lot of time in the UK, and I never had to use cash. You do not need to get money from an ATM. Your card will be accepted everywhere. For tips, you just add it to the bill. It's very simple, and you let them know what amount you want to add. Small purchases will always accept credit cards. Even church donation boxes now have the ability to accept Apple Pay and credit cards.

Worst case, if you need cash, just go to a bank ATM there. You'll get the best exchange rate. I definitely would not go to the post office or get your pounds in the U.S. before you go, or it will be quite pricey.

If you are more comfortable with cash on hand, find a bank ATM (CashPoint) near your hotel. Hopefully you have a card that doesn't charge fees.

Resist the urge to withdraw too much money. You will find most places prefer or only accept cards/Apple Pay/Google Wallet.