OK, so UK and I’m guessing not particularly mobile and likely to feel the cold a bit more than most.
I’ve used the Alpkit Filo down to around -15C just walking around, i.e. mildly active. Like most down jackets it will stand a light shower but not sustained or heavy rain. I’ve replaced it with a PHD Yukon which has a “waterproof” outer shell – I suspect it will deal with sleet. You are talking significantly more cost with the PHD over the Alpkit though.
A slight aside: down products seem to have significantly gone up in price over the last eight years or so, more so at the lower end – the Filo has gone from £80 to £200 for example, even the sale price is £140. (my older Filo also has higher fill power down than the current version.) Whether the prices back then were “low” and the market is now more realistic or it’s a response to increased demand I’m not sure.
BTW: Peter Hutchinson was the guy behind Mountain Equipment before he started PHD so some continuity for your dad there.