Depends how cold it is, but I'm normally in tights and a baselayer unless it's really unpleasant, in which case I'll stick on a windproof too. I've fell run in a Powershield softshell in windy conditions and -10, though, and that was definitely needed! I've also run in a full weight belay jacket a few times when seriously fatigued and cold and wet.
Managing the condensation will be more effective than a bivvy bag. Opening the vents and also, not wearing too much or overheating in the sleeping bag early in the night will help a lot. Don't cook in your tent either as the steam makes everything wet.
A bivvy bag is only really useful inside a tent if your tents leaks like a sieve, or you are really cold and need all the warmth you can get.
I'd persevere without the bivvy bag for now and just work on keeping the tent aired. With that sort of kit in the UK you shouldn't be cold and you might find that you're sweating early in the night then as the temperature drops you get cold due to the moisture in the bag.
I can wear merino for weeks, not wash it and it's fine. That's an inherent property of the fibre so garments ten years old will still have that magic ability. When it comes to synthetics, I've not found anything that can be worn for more than about 4 days on the trot, and after a few years of use most of them are limited to 2 or 3 days.
As an aside, anyone struggling with synthetic baselayers that stink whether they've been washed or not, there's a solution: wash them at 60 degrees. I've had ancient baselayers that stank the moment you got them out the washing machine or having been worn for just an hour, but one wash at 60 and they're good as new. Basically, 30 and 40 just isn't hot enough to kill bacteria in the garment. With most synthetics you're highly unlikely to shrink them at 60, and I've washed about ten baselayers at that temperature with no adverse effects.
It's an interesting jacket because in its current guise I would not buy it - it does not fit me, I don't like the pockets, and I don't want a hood. That's because I ride a road bike and don't tour. However, for some people I think it could be great. If Paramo made a genuinely tight-fitting one with conventional rear pocket and no hood I would definitely consider getting one for winter road riding and maybe commuting: the fabric's ideal and more suitable for cycling than most winter jackets that are either not breathable enough or not rainproof enough. Go on Paramo, you know you want to!
Yeah, fleece all the way for me. If you've a windproof layer with you too it's only really packsize and about 150 g that you are saving. Hiloft and gridded fleeces are amazing. I got through -20 C ice climbing by swapping my normal fleece (Eider thing, but basically an R1 with full length zip) for a Rab boulder one (Thermal pro).
Fleece is more durable, cheaper, deals with sweat better, and feels nicer than insulated garments too. One other thing you could consider instead is something like the Mountain Equipment Switch, which is super breathable but also insulated fairly well.