Bookworm - doesn't that limit you to larger bizjets? Your order of importance is interesting - I know you can do airways whereas I can't (yet) but for me it's usually been the icing level being below the MSA that stops me going in the winter.
OK. That and Bose-x's previous comment about IMC rating training in winter got me doing some homework.
I've looked at Dec 2003, Jan 2004, Feb 2004 -- the three months usually associated with the nastiest UK weather. I've taken the 00Z and 12Z Herstmonceaux soundings to find the freezing level and pulled out those days where it was at or below 2500 ft, which is a typical emergency safe altitude in the southern UK. I've then looked at the Gatwick TAF as being representative of the conditions in that area, and discarded the ones with no significant (SCT+) cloud below that level (the majority of them -- most low freezing levels are just cold clear nights/days). Of those approx 180 data points, I'm left with the following, for which I've examined the sounding in more detail to find the layer tops and lowest temp in the cloud:
00Z 01 Jan 2004
EGKK 010120Z 20014KT 150V230 9999 RA FEW012 SCT018 BKN024 05/02 Q1014 =
Freezing level 2200 ft Tops 3500 ft (-2 degC), though a possibility of freezing precip from higher layers.
12Z 27 Jan 2004
EGKK 271150Z 34009KT 310V010 9999 SCT016 BKN022 04/00 Q1008 =
Freezing level 1200 ft Tops at 3500 ft (-4 degC)
12Z 16 Feb 2004
EGKK 161150Z VRB03KT 9999 OVC016 05/02 Q1031 =
Freezing level 2300 ft Tops at 4500 ft (-5 degC, with above-zero inversion above)
12Z 20 Feb 2004
EGKK 201150Z 07012KT 9999 BKN020 02/M03 Q1020 =
Freezing level 800 ft Tops about 4000 ft (-8 degC)
00Z 22 Feb 2004
EGKK 220120Z 03013KT 9999 BKN018 04/M01 Q1012 =
Thin sub-zero layer, Freezing level 1600 ft warm air from 2500 ft up
00Z 25 Feb 2004
EGKK 250020Z 01008KT 9999 BKN010 02/00 Q1012 =
Freezing level 1400 ft, Tops 3500 ft (-5 degC), further layer at 8000 ft
12Z 27 Feb 2004
EGKK 271150Z 33011KT 290V360 9999 BKN024 03/M03 Q1000 =
Freezing level 800 ft, Tops 3500 ft (-6 degC)
Which of those would provoke you to scrub a trip in the TB20?