They've abolished peak trains on Friday afternoons on Virgin West Coast. (Probably because the whole of Euston was hanging around until 7pm and then attempting to leg it onto a rammed train).
It'll be Fri but I want the least hassle so if I can get out of work for a 16:30ish train I'd rather that than the later ones, which, if full, will leave me fucked and having to get super late ruining the point of booking a hotel up there.
From a London terminal? I would be careful with exactly when off-peakedness finishes for the afternoon, eg. the last train may be 16:23.