It all comes down to a 'reasonable length of time' between stays.
Now that's about as ambiguous as they come - what's reasonable? It's like this on purpose - it's up to the official at the border, as the purpose of this is to try and work out if you're trying to live in the states and just border hopping every 90 days, instead of visiting.
From the CBP website:
When traveling to the U.S. with the approved ESTA, you may only stay
for up to 90 days at a time - and there should be a reasonable amount
of time between visits so that the CBP Officer does not think you are
trying to live here. There is no set requirement for how long you must
wait between visits.
If you're worried they may think you're trying to live there, then you could bring extra documents as evidence - eg, your flight out of America back to the UK, or proof of your current employment and residence in the UK. Basically anything to convince them that you're not actually surreptitiously residing in the US :)
Note that going to islands or Canada doesn't reset it, as found on most US embassy websites:
"The Visa Waiver Program (VWP) enables nationals of certain countries, including Australia, New Zealand, and the U.K., to travel to the United States for tourism or business for stays of 90 days or less without obtaining a visa, if certain requirements are met. Under the VWP, time spent in Canada, Mexico, and adjacent islands counts towards the maximum of 90 days stay allowed under the program."
Short version - it doesn't matter, what matters is convincing the border officer that you're visiting, not living in the US.