a) You buy the slightly more expensive version of the common aftermarket headlights that are available which come with 4 adjustment screws in the back of the unit. They allow you to move the beam up,down,left,right and to flip the beam pattern to pass UK MOT law.
b) You buy the cheaper version which are set for left hand drive ONLY and cannot be adjusted to the extent of flipping the beam pattern and you'll have to buy little plastic clear lense covers which stop the beam from veering to the wrong side of the road, you'd need these if you drove from the UK to Europe anyway...
hope this helps...
FYI in the pic below in my sig I have the good kind of headlights bought from the states which I can flip the beam pattern on so mine pass MOT.
If you have OEM LHD headlights then you can use the internal adjuster to stop the beam kick up, just as you would with RHD for a trip on the continent. Not sure if that would pass an MOT though as I think the kick up is mandatory.