Charters direct to Edinburgh or Glasgow are the cheapest and fastest, but most only fly once a week, so if they don't fit your schedule you're out of luck. Round trip, direct flight Halifax - Edinburgh takes about 8 hours and ran me around $600 total... I think that was through either Flyglobespan or Thomas Cook.
I prefer flying a major airline as you get more room on the big 747 / 767's than you do on the smaller charter planes; I'm 6'2 and an 8 hour flight on one of those little Airbus charter planes just about kills me. Plus, you get in flight movies, free drink service, and seat backs which have a USB and power outlets so your laptop doesn't die halfway across, which makes the flight go much faster.
My usual route is Halifax - Heathrow - Edinburgh with Air Canada; the school tells new students to avoid Heathrow but in all honesty it isn't bad at all. As long as you pay attention to the signs and allow yourself 90 minutes between you're connection, you'll be fine. Total time travelling is 11-12 hours including waiting for your connection. If you book a few months in advance you can get prices that are nearly as good as a charter price - my last round trip ticket was $750 with taxes and fees.
