I remember (but not in detail) my first weekend away walking. I got a train to a stop near Ingleton. I then had a walk of at least 3 miles (I think) in the dark to get to the hostel for a Xmas break. Over the next few days I tried walking over Ingleborough and Whernside(?). Weather wasn't the best so I wasn't very successful but it was still a great time with some real characters staying at the hostel.
Another possibility is getting the train to Betws-y-Coed and staying there or seeing if there's transport to Capel Curig.
Weather not looking good for the early part of next week. Windy, wet and er ... windy.
40g/m2 of Primaloft insulation is reasonably warm at about 3 - 4C
60g/m2 of insulation is reasonably warm at about 0C.
100g/m2 of Primaloft insulation is reasonably warm at about -5C
130g/m2 of Primaloft insulation is reasonably warm at about -10C
170g/m2 of Primaloft insulation is reasonably warm at about -15C
200g/m2 of Primaloft insulation is reasonably warm at about -20C
However this same site doesn't make it clear if this is when active, inactive or slightly active. My own experience would suggest its when reasonably active but not actually walking for any length of time. i.e. bimbling round camp.
If inactive just sitting round I'd suggest dressing for at least 5C colder than the actual temperature. Since you are proposing 3 season activity none of this should be much of an issue.
All of the above figures are MY extrapolation of the figures published on the PHD web site. And so far they seem to be close to the mark.