The injuries you describe aren't sufficient to sustain a 'Section 18' (GBH with intent) charge. You've described 'ABH'. Take a look here:
http://www.cps.gov.uk...s_against_the_person/
(See paragraph 46 for the definition of 'ABH', paragraph 55 for the definition of 'wounding/GBH' and paragraph 65 for the factors adding 'with intent).
To use 'self defence' as a valid defence in law, the accused person must show that he could not simply flee from the other person, and that he could not have prevented an assault upon himself by using less force than he actually did. The location of the incident is completely irrelevant.
The sentencing guidelines for ABH, GBH and GBH with intent can be viewed here:
http://sentencingcoun...ine_-_Crown_Court.pdf
Chris