The law when overtaking
1.5m is a minimum safe distance for overtaking in slow moving traffic.
If you cannot allow the minimum distance, do not overtake until you can. At speeds of 30mph or above, the Highway Code recommends a car width may be needed to overtake safely.
states “give cyclists at least as much room as you would a car when overtaking”.
states "drivers should take extra care and give more space when overtaking cyclists in bad weather (including high winds) and at night."
If a cyclist looks over their shoulder while you are following them it could mean that they intend to pull out, turn right or change direction. Give them space and time to do so.