London is a work destination too, not just holidays, although it's a nice city to visit year-round. I was there last winter.
The only thing is, if you think Halifax is dark.. Halifax is 44 N and London is 51 N. The constant London rain stereotype is exaggerated but it's very overcast. Even so, London has plenty of nightlife for 3 pm and later.
Sometimes when people visit NS from the UK, including in winter, they remark on how many more sunny days there are.