There are several filghts a day from Gatwick with British Airways and Virgin Atlantic. Virgin Atlantic and BMI also fly direct from Manchester. Connecting flights are available from Scotland via Gatwick and Manchester. Flying time is around 8.5 hours.
Fares change constantly so it's worth checking the airlines' websites for latest prices. Frequent flyer programmes can offer great discounts. For example - Virgin Atlantic flying club offers up to £200 off per flight using 2000 air miles - so it can be worth simply buying miles.
There are daily flights from many major airports, with connecting flights from all over USA and Canada. Flying times are around 5 hours from New York and 5.5 hours from Toronto.