The critical thing is the beam pattern and wattage of the bulbs. Colour of bulb is also taken into account.
You can fit any BS/CE approved headlamp units to any car. If going to a different bulb type, particularly HID then you also need washer jets and auto levelling and as long as the beam pattern is ok then you'll be fine.
So as long as the light pattern conforms to the MOT testers guidelines you should be ok.
NB fitting LED bulbs from conventional filament bulbs in standard light housings can alter the light output pattern and will need adjustment.
Have a chat with your local MOT tester as much is up to their interpretation of the law and guidelines.
IIRC the high beam reflector will not provide the correct low beam pattern, this is why they are fitted to different reflectors. It's not just about the wattage in the bulb but the way the light spills out onto the road.
Changing the reflector as well as the bulb is the only way to go if you want to stay legal.
I'm sure someone does a decent T5 headlight conversion kit that would be suitable, and they will have sorted out all the issues with the beam pattern.