Much of the improvement compared to the 00z runs comes from the western N. Atlantic trough being aligned well to throw some warm air advection up the west side of the mid-Atlantic ridge, allowing it to hold on even with shortwave lows cutting through it.
The JMA 12z hasn't followed this trend so makes a good reminder of what we don't want to see.
Shorter term, regardless of what S. Murr has posted (based on GFS...), WRF-NMM continues to trouble me with it's positive dew point values for Wednesday night across much of Cornwall, Devon, CS and SE England. Not quite as bad as the past couple of runs, with some snow shown from a bit of heavier precipitation for a time, but still suggesting that accumulations would be slim to none.
While an issue with modelling the effect of the bordering ocean and seas on dew points may be having a significant impact, there is also a significant disagreement in terms of wind direction; WRF-NMM is showing a SW wind 3-6am when GFS has a WNW wind. That slight difference could be critical - in fact I remember yesterday's NMM 06z had snow right down to the coast in a WNW wind.
So the usual uncertainty reigns supreme and keeps me guessing until the potential is on my doorstep. As usual, the instinctive response is to expect nothing much, roll the eyes, and carry on in 'business as usual' mode.
So looking further ahead again, Friday is looking cold but not markedly so, with air temperatures as high as 6*C in the south. Dew points of near zero may give a slight 'continental' feel to the conditions with any well-shaded frost or lying snow able to hang on through the day. Any showers getting inland are likely to be wintry in nature - so a mix of rain, sleet, soft hail and snow, varying between locations. Windward coasts may see a lot of showers but these appear likely to fall as rain in England and Wales, snow reserved for Scotland.
Saturday may see an area of higher DPs drift down across the UK, which will result in some unwanted thawing of any remaining lying snow should it transpire. GFS makes a lot less of this on the 12z compared to the 06z, but it still looks to be present on the ECM 12z.
The culprit is a secondary low developing over the UK, which mixes out the low-level cold.
Sunday now looks increasingly cold from the north, but beyond that the details are impossible to pin down aside from the lowest temperatures being in the far north and the highest in the far south.
Overall, the Wed-Sat period is currently looking likely to be a very tame cold spell across low levels in the south, unless the convection early Thursday turns out to be heavier than the models currently indicate, in which case there is some scope for rapid accumulations of 2-5cm in spots that get lucky.
As usual, the prospects improve as you head north, though from the looks of things not by a lot until you're up near or in Scotland.
I will admit, taking such a critical, level-headed approach does become tedious at times like this 
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T-Max: 30.2°C 9th Sep (...!) | T-Min: -7.1°C 22nd & 23rd Jan | Wettest Day: 25.9mm 2nd Nov | Ice Days: 1 (2nd Dec -1.3°C in freezing fog)
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