Money

The standard advice is to wait until you get there and access a bank associated ATM for the best exchange rate. Having said that we are going this summer as well and I will get about 150-200 pounds from my local bank. The current exchange rate is pretty good and the hit you take (poor exchange rate compared to what you can get in the UK) on that small amount is pretty minimal compared to the price of your entire trip, I like to land with currency in my pocket. I use that to pay for transport, snacks, first day incidentals. When we are checked in settled, rested, I visit an ATM. An alternative is to travel without foreign currency and get it at an airport ATM. I've done that and don't like that extra step when I'm tired, having to clear passport, get to the hotel. But, to each his own. Wells Fargo in Cherry Creek has foreign currency.

Alan's right, get a little for that first day, maybe 25-50 pounds would do it. Then use your debit card at ATMs. Here's a lot of good info from this website.

I agree get a small amount and small bills if it will make you comfortable having money when you get there and not having to find the atm right away.
But hands down you save more money by taking it out of a local atm

How come we haven't seen you at a third Saturday get-together in Denver?

I always advise people to have a few pound or euro on hand when you get to Europe so you don't immediately have to find an ATM. The best place to get foreign currency in Denver is Wells Fargo. You'll pay about 5% over the Interbank rate. If you are using a major bank (US, Chase, Wells, et al) debit card you will pay 4% over anyway. Go to someone like Credit Union of Denver or Charles Schwab for a better reate.

The cheapest and most convenient way to obtain local currency is via a debit card at a bank owned ATM - bar none.

What MrsEB says about the cost is true. However, it's not a bad idea to have a little bit of local currency when you land. If you want to do that, just realize that your bank will not give you a good exchange rate. (It may also add a flat fee.) Because the rate will be poor, do not get more GBP than you need before leaving home. Normally, 100 GBP would be more than two people would need to get from the airport to their hotel.

Do you really need pounds ?
Cabs are advertising .....All black cabs now take MasterCard,.... saw it today

Almost all the cash machines at Heathrow are now Travelex. There are a few others but hard to find.

Good advice in the above posts about getting more cash from machines if you need to.
We always like to arrive in England with a decent amount of cash (pounds).
Yes, get some pounds from your hometown bank before you leave for England.
For a recent trip, my husband and I each got 300 pounds, ordered from our bank two months before our trip.
We requested that the bills be in low denomination notes-- £5, £10 and £20.
We each wore a money belt and stored our valuables there--money, credit cards, passports.
https://store.ricksteves.com/shop/p/moneybelt
A small amount of cash was kept in a front pocket.
We mainly used a credit card for our purchases, so we came home with a lot of cash which will be used during our next trip.
We did not visit an ATM while we were in England.
Most of the cabs in London took credit cards; however, one wanted cash.
The cabs in Salisbury and other small towns did not accept credit cards.
Do let your credit card companies know that you will be using your credit cards in England while you are on your trip.
You will want to buy an Oyster card if you will be staying in London for awhile.
The machine for buying Oyster takes cash or credit card; either one.

Travelex at UK airports is annoying but it can be convenient to get some cash while waiting around for your luggage.