London is typically 3 hours ahead of Rio de Janeiro. When it's 10:00 AM in Rio de Janeiro, it's 1:00 PM in London. During British Summer Time (late March to late October), the difference increases to 4 hours.
Brazil suspended daylight saving time in 2019, while the UK observes British Summer Time. Rio de Janeiro uses Brasilia Time (BRT/UTC-3) year-round, making the time calculation relatively straightforward outside of UK DST transitions.
Optimal meeting window: 9:00 AM - 2:00 PM Rio time (12:00 PM - 5:00 PM London). This catches afternoon business hours in London while being morning to early afternoon in Rio.
For UK morning meetings starting at 9:00 AM, participants in Rio would need to join at 6:00 AM, which is quite early. Consider afternoon London time for better overlap.