Confluence's English language pack is named "en_GB", while JIRA's is named "en_UK".
When the JIRA Activity Stream gadget is embedded into Confluence, it displays an error because it is looking for an "en_GB" language pack in JIRA, which does not exist.

"The resource http://penny:8080/s/472/1/2/_/download/resources/com.atlassian.streams.streams-jira-plugin:date-en-GB/date-en-GB.js, cannot be found."

There are three issues here:
1) JIRA and Confluence are not consistent in their naming of their English language packs.
2) UK is not a valid country code. According to ISO 3166, the correct country code for United Kingdom is GB. http://www.iso.org/iso/english_country_names_and_code_elements
3) The Activity Stream gadget does not fall back to the default language when a matching language pack is not found. This may impact people who write their own translations of JIRA.