If you're a US citizen, then carrying a driver's license for ID is the way to go.
But for some of us (europe and UK) it's not always as simple, especially for those of us who still have paper DLs that might be recognized at the airport for car rental, but much less by CMs and TMs (not to mention that not everyone has a driver's license.)
usually a DL (even a paper one) will work in conjunction of a laminated color copy of the passport, or a scan/picture of the passport on a smartphone.
but if, for some reason, you can fit in any of the above cases, unfolrtunately, you will need to consider bringing your passport ... we used to have to do that, I know what it fells
regarding ID checks at the gate, the very first time we used a 14 days UK ticket, we were asked for ID, but it's really down to the TM, some won't let you proceed without seeing your ID, some will look at you like you're an alien if you even hint you're willing to show your ID ... pot luck.
We also were asked twice for ID when our fingerprints failed to scan ( using APs) DW, DS and I would not manage to have a green ... some TM took my DL and came back 5 minutes later then asked for my wifes (sure couldn't ask for both at the same time, right ? ) but she doesn't have a DL, and she didn't have any ID ... took us forever to enter the park, but we eventually did