I have found a better solution than to keep a duplicate version of en.yml and simply change the $ to £.
There is a plugin that lets you only override the values you require:
http://github.com/javan/rails-i18n-translation-inheritance-helper
config/environment.rb
config.i18n.default_locale = 'en-UK'
and then create:
config/locales/en-UK.yml - for special cases
en-UK:
  number:
    currency:
      format:
        unit: '£'
        format: '%u%n'
config/locales/en.yml - for all English translations
en:
 btn_submit: Submit
This works a treat and will also mean that I don't need to maintain the file apart from any special cases like above.
View
=t 'btn_submit' #Submit
=h number_to_currency(@price, :precision => 0) #£1,000
Hope this helps others as it took a while to find a solution.