This makes sense when it is realised that the Celts of Central Europe originated from the northern tribes, before their culture mixed with that of the Mediterranean lands and the aboriginal races of what is now Britain. It is well-known that due to difficult access of the remote northern areas (i.e., for the Romans), the Nordic/Saxon cultures retained a greater degree of purity within their customs and language, so this may also be a contributing factor.

Hmm, Germanic poses a problem as a branch within IE languages. Fully a third of its vocabulary is not traceable to PIE or the other IE languages. This may be because they preserved this word hoard, but it may also be because they absorbed some locally spoken non-IE languages. Professor Theo Vennemann has suggested that Proto-Germanic may have been a creole spoken which developed when Old European (his cover term for aboriginal, non-IE languages in Europe) and IE collided. Not sure I'd go that far, I find it hard to believe that pre-IE Europe was unified in language.

I've never seen a reference to a northern origin for the Celts. I thought they came in off the Asian steppes with the other IEs. You might want to look at some of the recent scholarship on the "problem of the Celts". See The Celts: A Short Introduction, The Ancient Celts, or Iron Age Britain, all three by Barry Cunliffe, Atlantic Celts by Simon James, or Celts: Origins, Myths, Inventions by John Collis. (James and Collis are amongst the so-called Celtoskeptics, of which see Professor Patrick Sims-Williams' "Celtomania and Celtoscepticism." Cambrian Medieval Celtic Studies 36 (1998): 1-36.)

Ceci n'est pas un seing.