How can I run plumbing from house to a nearby new office 1.5m away?
This is a very localized question.  If this situation happened in Africa it would be perfectly fine to run the line straight to other house.  The whole point is to have the pipes running through an area that won't freeze.

It will really be up to your local building code.  You want to ask your local inspectors.  I have set up plumbing for exterior garages in the midwest and I have been told between 2-3 feet deep.  But I am not sure they cared that much since everything external I have done was garage related.   The whole point is to get the water lines below the freeze line.  I would think that the UK would have similar requirements as midwest US (maybe we are colder for longer periods).