Thank you all for your inputs. I looked up in the mentioned document and this is what I've found in CAP 168:
1.2
Periods of display of Aeronautical Ground Lighting
1.2.1 Aeronautical ground lighting should be displayed at least 15 minutes before the estimated time of arrival of any aircraft and until at least 15 minutes after the actual time of departure of any aircraft as follows:
By day: High intensity systems, where installed on the runway to be used, whenever the visibility is less than 5 km and/or the cloud base is less than 700 ft.
By night: Irrespective of weather conditions.
Now, you can argue what the systems mean but my opinion is that, as everything is costs vs. safety and being pilot I beleive having the all available light on helps make safer landings. And finaly I remember well that at airports like EGLL, EDDF, EHAM... lights are ON all the time. Please correct me if I'm wrong.