By Andrew MacAskill LONDON (Reuters) – Britain is working on a package of military support and economic aid for Ukraine as the threat of a Russian invasion grows, a government spokesperson said on Sunday. Prime Minister Boris Johnson will make a trip to Europe later this week to build support to end the standoff with Russia. Although there were no details of where Johnson would visit, his office said he wanted to engage more with Nordic and Baltic countries. “The crisis on Ukraine’s border has reached a critical juncture. All the information we have suggests Russia could be a planning an invas…