Automatic teller machines (local name: cash machines) are everywhere in United Kingdom. Pretty much anywhere you look there will be an ATM. United Kingdom has one of the highest ATM densities in the world.
You do not need a chip & PIN card to use an ATM — your standard magnetic card will work fine.
The exchange rate you'll get on an ATM is usually the best one you'll get. You’ll pay fees, but you’ll still get a better rate than you would exchanging cash at a bank or exchange booth.
When possible, withdraw your cash from ATMs located outside banks — a thief is less likely to target an ATM near surveillance cameras, and if your card is swallowed by a machine you can go inside for help.
Some banks have their ATMs in a small entry lobby which may be locked when the bank is closed. In that case, there is a credit-card-size slot next to the door. Ideally you should insert/swipe your debit or credit card in this slot and the door should automatically open. However, note that this slots are frequent target of skimmers. Try to use any other card you might have with a magnetic strip on it (like library card or personal ID) and usually door will still open.
Some independent ATMs will try to offer you dynamic currency conversion (DCC for short or also called Cardholder Preferred Currency or CPC for short) which first converts the amount to your home currency and then proceeds with the transaction - the benefit being that you can see the amount in your home currency and therefore there is no need to look up/remember exchange rates. Do not do it! You will end up paying more because of lousy exchange rate. If ATM offers to "lock in" or "guarantee" your conversion rate, choose "Proceed without conversion". Sometimes it will say "You can be charged in pound: Press YES for pound, NO for euro." - always choose the pound. DCC is also a problem while using credit cards.
Stay away from independent ATMs like Travelex, Euronet, Moneybox, Cardpoint, and Cashzone. They have higher fees than normal ATMs and usually will offer you DCC.
Usually the bank charges you a flat fee per transaction/withdrawal which means that is cheaper for you to visit ATM fewer times but withdraw larger amounts.
Skimmers are devices placed to the card reader by criminals in order to capture your card information and PIN number. They are usually placed on the ATMs, gas pumps, or other self-service terminals which receive a lot of traffic. They are usually passive devices which means that your card will still be processed by actual ATM reader as usual - in order for you to not suspect anything.
There is no bullet-proof way to be secure, however following three steps below can dramatically reduce the chance of you being the victim.
Before you pull out your card, visually inspect the ATM - does anything look out of ordinary (although this may be a bit harder to do for international ATMs) or is damaged? Is the card slot sticking out dramatically? Is there something going around the keypad? This may be the sign of a skimming device.
Even if you don't find anything unusual upon visual inspection, still try to wiggle a card slot reader and a keypad. The real ones should not move/wiggle at all.
The last line of the defense is to cover your PIN with the hand while you are entering it to prevent the criminals by recording it via hidden camera.
Even if you follow all the steps properly - there still may be a chance of a placed skimming device like using overlay keypad over the real one (annulling your hand cover while entering the PIN), using whole ATM mask overlay instead of just the card slot (annulling your wiggle inspection) etc. After all - if something doesn't look right (even with just according to your intuition) - simply find another ATM (preferably inside the bank, or outside with surveillance cameras) if possible.
To learn more about skimmers see All About Skimmers - Krebs on Security.