The research team was collecting data on the plume
Scientists from Wiltshire who have been making test flights into the volcanic ash cloud say the decision to close UK airspace is not an over-reaction.
Experts from Swindon-based Natural Environment Research Council (Nerc) flew over London and tracked the plume's edge near East Anglia.
Flying just below 10,000 ft (3,000m) their instruments recorded "heavy gritty particles" at about 8,000ft.
The data is used by the Met Office to help in forecasts.
The first flight involving a Nerc team took place last Thursday when the ash cloud was first spotted.
A second flight took off from Cranfield airfield in Bedfordshire on Saturday.
A Nerc spokesman said: "On a basis of what we've seen, its (the ban) the correct decision.
"I don't believe there is any doubt and the decision is not an over-reaction.
"Aviation is a very unforgiving business and I don't believe you can do anything but err on the side of caution."
Their Dornier 288 plane tracked the edge of the plume over East Anglia and the North Sea, and towards the Dutch coastline.
Flying at just below 10,000 feet, research instruments identified three distinct layers of volcanic residue.
Heavy, gritty particles seem to be sitting at around 8,000ft (2440m) , while lower down in the atmosphere there are sulphurous chemicals and finer dust particles.
The plane is modified with small holes which suck in air, enabling experts to analyse its gas content.
The modifications also mean the team can closely monitor how close to the plume they are, enabling the plane to fly safely near to it.
The grounding of all non-emergency flights from England's airports - which began on Thursday morning - will remain in place until at least 0100 BST on Monday as the plume continues to drift across Britain.
The ash cloud resulted from an eruption in the Eyjafjallajoekull area of Iceland.
Earlier on Sunday it was reported the Dutch airline KLM had flown through the ash cloud without the test plane being damaged.
The carrier says it plans to carry out more tests on Sunday.