By way of an update - I nabbed a Rumble LT25 (the digital one, with an 8in speaker) at a fair price from the bay. It absolutely is fine for my purposes. Sure it wouldn't be much cop when playing with a drummer, nor will it reveal the subtlest tone nuances... But it's absolutely fine to noodle on the couch along with music from the stereo, at sensible volumes. And truth be told it's actually bloody impressive for a hundred-odd quid: it's light, looks pretty classy, the effects are surprisingly ok (including the octaver) once you move away from the presets, and it's nice to have a few amp models to choose from (again, the B15 is surprisingly good with flats, especially through headphones).
All in all, a happy camper! I might upgrade to a Rumble 40 one of these days but as someone who plays the vast majority of the time through headphones, I don't think I'd get much from it.
I didn't get to try other "beginner" combos (Rocket RB108 and the likes) but can definitely recommend that one for home practice.