。
He's just about to leave for London.
Literal
He [topic-は] precisely London [toward-へ] depart [about to-ようとしている].
ロンドン written in katakana follows the standard convention for foreign place names. へ marks direction — 'toward London' — and is the more idiomatic choice than に for departure-style motion to a far destination. The non-past form ~ようとしている (rather than past ~ようとしていた) puts the imminence in the present — he's standing at that exact threshold right now.