Phoenix is 7-8 hours behind London, depending on the time of year. Arizona (except Navajo Nation) does not observe daylight saving time, making the difference variable.
During UK summer (BST, late March to late October), Phoenix is 8 hours behind. During UK winter (GMT), Phoenix is 7 hours behind. This is the opposite of most US cities!
Optimal meeting window: 6:00 AM - 9:00 AM Phoenix time (1:00 PM - 4:00 PM or 2:00 PM - 5:00 PM London, depending on DST). This catches afternoon business hours in London.
Be extra careful with DST calculations - Phoenix stays on Mountain Standard Time year-round.