Ok, so I know that I have to leave the US within 90 days on the VWP (I'm here now and my ticket back to the UK is July 25th). Canada, however allows me to stay upto 6 months on my UK passport... and I'd rather be there than the UK. If I leave the US, fly to the UK (therefore leaving the USA entirely), and then a week later fly into Canada, will I have any issues? ..And 4-5 months down the line, I'll probably travel from Canada to Washington where my girlfriend lives. I certainly won't be attempting to enter the US for at least 4 months. Any problems I'm likely to face?
If I leave the US, fly to the UK (therefore leaving the USA entirely), and then a week later fly into Canada, will I have any issues?
Nobody can tell definitively if you will have any issues. People with similar travel patters have been allowed in, and similarly some have been turned away.
It depends on the immigration agent at the border. That said 3 months in USA, 1 week in UK, and then a flight to Canada is not strange however if you indicate to the Canadian officer you're looking to spend about 5 months in Canada, it would clearly show the UK is not exactly your home base any more and your ties to it are tenuous, e.g. no job there, apartment etc.
And finally even if Canada let's you in, chances are high you will be turned away by the US CBP at the border. In eight months you would have spent one week in your home country UK, that's not typical coupled with a girlfriend in the USA? Very high probability you get turned away in these times.
What you're trying to do is akin to satisfying the letter of the law (by completely leaving the American continent for one week) while violating the spirit of it, if you get my drift.
My sources of reference would be numerous Canada Border Security shows on YouTube and extensive immigration experience.