In the UK, how do I run wiring from indoors onto a balcony for a light?
The Building Regulations (which determine whether work is notifiable or not) vary according to England, Wales, Scotland or Northern Ireland. Notifiable work must be notified to the local authority, either by a Building Regulations application or an electrician who is a member of a scheme which allows members to self-certify and register their work. Anyone can carry out electrical work; it is only a requirement to notify the work that may apply.


The latest version of Part P for England and Wales was introduced on 6 April 2013, the range of works requiring notification has been reduced. 


  Notifiable works are:
  
  
  The installation of a new circuit
  The replacement of a consumer unit
  Any addition or alteration to an existing circuit in a special location (bathroom, swimming pool or sauna)
  
  
  All other work is considered non-notifiable, namely additions and
  alterations to existing installations outside special locations,
  maintenance, replacement and repair work.


Part P England and Wales - Frequently Asked Questions

See also Approved Document P: Electrical safety - Dwellings (2013 edition) (PDF 2.33 Mb)

Other Building Regulations also apply, including Energy Efficiency - you may not be able to install a high-power halogen light. 


You should use a fused connection unit adjacent to the power socket. It is your choice whether to include a switch or not, but it would probably be advisable. 
You should use an appropriate cable for external use, such as NYY-J or "Hituff" cable. This can be run on the outside on the wall without conduit, provided it is not going to be subject to physical damage (so if it is running near ground level it may require a protective conduit or cover). It is available per metre from some electrical suppliers, including TLC


You may require a weatherproof junction box and glands on the outside to connect between the light fitting and the circuit wiring. 

Note that the latest Amendment to the Wiring Regulations now requires all wiring to have sufficient metallic fixings that the wiring remains sufficiently supported in the event of a fire. This does not mean that every fixing must be metallic, and short lengths of wiring may not need any.


You do not require a switch on the outside.
The Ip rating depends on whether the light may be exposed to any form of jet washing.



  The minimum IP rating you should look for in a garden light is IPX3
  (normally IP43), which protects against rain or spraying water at a
  60° angle from vertical. Choose an IPX4 (normally IP44) rating for
  exposed areas.


IP Ratings – A brief guide to Waterproof Lighting

This answer is not a complete guide to how to install an outside light. Good workmanship is required.