Letter to Diekirch in 1888—Registered in London, Re-registered in Belgium? Why?

2d QV Inland Registry Envelope Uprated With a 2½d QV Definitive Seething Lane B.O. E.C. 25 Jan 1888 Registered London 4E 25 Jan 1888 Luxembourg-Gare [b/s] 26 Jan 1888 Diekirch [b/s] 26 Jan 1888

This is the first example of incoming mail I’ve seen from this (or any) period with re-registry in Belgium? What purpose did re-registration serve? Perhaps the Belgian registry label was applied due to the absence of a London registry label (and number).

Incoming Mail from England to Joseph Schock, editor of Les Petités Affiches du Timbrophile, in 1898

London E.C. 12 24 Dec 1898 Luxembourg-Gare 25 Dec 1898

Please send sample copy of “Les Petités Affiches du Timbrophile.”