Back to the first question... yes, the european headlight will fit your car.
The only thing you need to change is the harness plug. On USA car, there is
only 4 wires. Euro headlight got 5 wires... the extra wire is the city
light... that little light above headlight. Other 4 wires are the same but
arranged in a different order.
Now, your later post tells me something about UK light law... I am not
familiar with that. I know for a fact that Euro blinker light is strictly a
blinker light... it only lights up when you turn on signal. Their side light
is also "Orange" color rather than yellow-orange in USA... the orange match
the tailight orange signal light.
To determine which one is the side market always on circuit... just use a
voltmeter... there will be three wires... one is brown... which is ground...
one will be side marker and one will be turn signal. So when you get 12V
with light on... that's the wire you need to remove from the socket and
reroute it to your separate side marker... which I believe is that 5th wire
on the headlight.
Like DAS said, it is tricky to see if the headlight are in fact UK lens...
the only difference is the glass... you can buy new glass that conforms to
UK law. It is very easy to tell when you look at the glass... You will see
in the middle of the headlight portion... you will see a line and then you
will see the angle upward at about 40 degree angle...
USA and other countries that drive on right hand side of the road... you
will see that slant go up on the right hand side of the car. On UK, Ireland,
Japan and others... you will see that slant go up on the left hand side.
I am glad you are now understanding that those flat glass auro headlight...
does fit on all W123. I change al my MB to euro headlight when I buy the
car. It is the way the car should look and I get better light on the road.