London is typically 8 hours behind Singapore. When it's 5:00 PM in Singapore, it's 9:00 AM in London. Singapore does not observe daylight saving time, while the UK does.
During British Summer Time (late March to late October), the difference is 7 hours. During GMT (November to March), it's 8 hours.
Optimal meeting window: 8:00 AM - 11:00 AM London time (4:00 PM - 7:00 PM Singapore). This catches late afternoon in Singapore while being morning in London.
Many British banks and law firms with Singapore offices schedule calls during this window.