I dont know enough about effects of major SSWs, but from reading posts on NW in last few years from likes of Chinio they absolutely play havoc with model output runs for days and weeks.
The forecast LP for later in the week (4/5 days away) could go absolutely anywhere. From barrelling through West Country thru Midlands and Northwards to not even barely touching the South Coast.
Yes....that extreme difference. Genuinely.
I've seen these systems be forecasted to hit 'us' head on...and then an updated forecast some 24 hours later takes it south in N France.
The track of the end of week Low will prob chop&change over next 3 days. That dartboard version from GFS will modify; they always do nearer the 'hit' time.
'We' will either
(i) Get buried on Friday
(ii) Snow to rain
(iii) V little precip
OR
(iv) Nowt at all...Yes nothing, as it skirts into N France.
Pains me to write it, but wait til Thurs morning to see forecast.