I've a m1 macbook pro 14" and a LG 27UK670-B display connected via USB-C to the mac. Connecting via USB-C leads to the macbook charging it's battery (taking power from the display). I'd however prefer using the original magsafe charger while keeping the USB-C for simplicity. Can this be accomplish or should I drop the USB-C connection and simply go for the HDMI option? TYVM.
While I have not seen this documented anywhere to confirm this behavior I believe it safe to assume that the new laptops with MagSafe operate much like previous laptops with only USB-C for power, the laptop will draw power from the more powerful power supply. Connecting two power supplies will do no harm, the laptop will simply draw power from whichever source reports itself as supporting the greater capability.
Is there some concern that the connection of two power sources would do harm? I'd think that Apple engineers would know it is certain people would connect more than one possible source of power and put in protections to prevent harm. There's no reason you should have a concern on connecting both the USB-C display and the MagSafe power supply.
You mention a preference for USB-C for "simplicity", I don't understand how USB-C is any simpler than HDMI. Is it because you move your laptop often and prefer the "flippable" USB-C? Are you using the display as a kind of dock to connect other devices? Whatever the case feel free to use either port to connect your display as this should have no impact on your ability to use MagSafe for power.