a pair of Paramo Aspira salopettes would fit the bill for both - though you are looking at £200... TBH i find the sallies great for climbing, but they /can/ be a bit warm for straightforward winter walking, particularly if you also wear something like a buffalo top.
Paramo Cascada trousers (£80 from most internet retailers) would handle the walking bit with aplomb, and TKMAXX is a great location for finding skiing salopettes - you'd probably get a decent set for £50 - so by buying two items you would get kit better suited to their respective activities and save a bit of cash.
IMHO the two tasks have radically different demands, so finding one bit of kit that does both well is going to be a bit of a nightmare.