I am planning to travel to Vancouver to look for work. I work in IT so I do not need a LMO as my occupation is covered by the national confirmation program.

As a UK citizen I can have my work permit processed at a port of entry.

So once I have a job offer and my paperwork is in order, can I just cross the border into the US, turn round, and have my work permit processed at the border ?

As a UK citizen, you would be eligible to have a work permit in this category processed at a port of entry on the way back from the USA.