You're extraordinarily unlikely to be asked to show how much money you have available.
As a British citizen, the general process for you entering the USA at an airport will be to go to an Automated Passport Control kiosk, insert your passport, scan your fingerprints, touch the customs declaration questions on screen, then take your passport and receipt to the desk where an officer will look at them, ask you a couple of brief questions, and send you on to baggage claim.
If your receipt prints with a big X, then you are probably going to have a long conversation with CBP. In that case, you might be asked about finances, depending on the specific reason for the referral.
So, the question is, how do you demonstrate how much money is available to you?
- You can show the cash in your pockets, if any.
- You can print bank statements and credit card statements.
- You can log in to your accounts using your mobile network or the airport's WiFi and show your current balances. (But ask first if they specifically want you to do this Using your mobile phone is not normally allowed in the immigration control areas.)