Where do I measure a building's height on uneven ground (for UK Planning restrictions)?
Edit to answer new question:

It appears that currently you'd measure the height from "the highest part of the 
adjoining land.", which would mean that you'd measure from the up side of the slope. Source. 

So your building; as planned, is in compliance.


  ‘Height’ - references to height (for example, the heights of the eaves on a house 
  extension) is the height measured from ground level.
   Ground level is the surface of 
  the ground immediately adjacent to the building in question. Where ground level is 
  not uniform (e.g. if the ground is sloping), then the ground level is the highest part of 
  the surface of the ground next to the building. Source


Original answer:

The verbiage of the laws is very important.  After some research, I think I've found what you're referring to.


  Outbuildings are considered to be permitted development, not needing planning permission, subject to the following limits and conditions:  
  
  
  Outbuildings and garages to be single storey with maximum eaves height of 2.5 metres and maximum overall height of four metres with a dual pitched roof or three metres for any other roof.
  


Which means for a flat roof shed, you can have an overall height of 3m, with an eaves height of 2.5m.

Furthermore:


  If you want to put up small detached buildings such as a garden shed or summerhouse in your garden, building regulations will not normally apply if the floor area of the building is less than 15 square metres and contains NO sleeping accommodation.
  
  If the floor area of the building is between 15 square metres and 30 square metres, you will not normally be required to apply for building regulations approval providing that the building contains NO sleeping accommodation and is either at least one metre from any boundary or it is constructed of substantially non-combustible materials.
  
  Building a detached garage of less than 30 square metres floor area would not normally need building regulations approval if:
  
  
  the floor area of the detached garage is less than 15 square metres.  
  the floor area of the garage is between 15 square metres and 30  square metres, provided the garage is at least one metre from any boundary, or it is constructed from substantially non-combustible materials.
  


Quoted text comes from the Outbuildings section, of the Planning Portal.

NOTE: Check with the appropriate government agencies before starting this, or any other project.