The DS ain't region locked, so yes, you can play PAL (UK) games on an NTSC DS, and vice versa.
However, DS games that use special features of the DSi or 3DS (like the camera, for example) are indeed region locked, so be wary if any of the DS games you intend to purchase have any special functions with the newer DS'.
And just for the sake of stating it, the 3DS is 100% region locked.