Are there any ideas as to what the English landscape was like before the arrival of the Angles?

The Angles, along with the Saxons and the Jutes, probably started arriving in Britain around the middle of the 5th century, some 50 years after Rome abandoned its northern-most province. They would have mostly seen a landscape with many features of the late Roman period (described below) in a state of decay, alongside Celtic Iron Age dwellings.

By the time the Romans arrived in Britain, much of England was already deforested though there were significant regional differences (this link is a download), ranging from around 15% forested area in central England and East Anglia to around 40% in upland northern England (largely modern-day Cumbria). Farming was both arable and pastoral with most people living in Iron Age-style roundhouses, which were made of timber and thatch, dotted across the landscape of much of England.

Source: Chris Gunns [CC BY-SA 2.0 (https://creativecommons.org/licenses/by-sa/2.0)], via Wikimedia Commons

The landscape the Romans found was one of cultivated fields and pastures, scattered farmsteads and settlements, and surviving islands of managed woodland.

The Romans eventually criss-crossed this landscape with roads, beside which many villages and towns developed. These often had rectangular houses and shops fronting onto the road.

The remains of a Roman villa at North Leigh, Oxfordshire. Early Anglo-Saxons would probably have seen more than this as, over the centuries, people removed stones to construct churches, walls and houses. (Image is Copyright Richard Croft and licensed for reuse under this Creative Commons Licence.)

Probably no more than 10% of a population estimated to be between 2 to 3.5 million would have been ‘agriculturally non-productive’ during the late Roman period. This would have declined as

Towns and villas were falling into ruin within a generation [of the Roman departure]. It can be argued that the Anglo-Saxons, who arrived in numbers some decades later, came into a political and cultural vacuum – although many of the people were apparently still there, farming the landscape, albeit probably in smaller numbers.

Source: The British Museum, 'Roman Britain'(pdf)

It is also believed that the population declined during the centuries following the Roman departure, perhaps by as much as half. Further,

With the disappearance of the Roman system the population of Britain would have reverted totally to a subsistence agriculture mode. We may thus expect for the area that is now England to find a settlement pattern made up of farmsteads for nucleated or extended families practising agricultural strategies designed to yield little in the way of surplus above that necessary to perpetuate the crops and herds.

Source: A.S.Esmonde Cleary, 'The Ending of Roman Britain'

However,

Squatter occupation..... seems to have continued in a number of towns and it is a measure of the skill of Roman planners and engineers that only two major Roman urban sites are unoccupied today, Silchester and Wroxeter...

Source: Michael Reed, 'The Landscape of Britain'

Source: Lotroo / R. Botek; Изработено от Потребител:Lotroo (Own work) [Public domain], via Wikimedia Commons

The Roman road network,on the other hand, would have been evident:

One important survival was the road network, which formed the skeleton of communications in Britain until the 18th century.

Source: The British Museum, 'Roman Britain'(pdf)

When the Angles, Saxons and Jutes started to settle, the generally accepted view is

the taking-over by the Anglo-Saxons of a Romano-British site or social feature and its adaptation the better to respond to Anglo-Saxon priorities.

Source: A.S.Esmonde Cleary

In practice, this meant that many buildings were either (1) torn down so the materials could be used elsewhere, (2) put to some other use or (3) simply abandoned.

Other sources

P. H. Sawyer, 'From Roman Britain to Norman England' (2nd edition, 1998)

Simon T. Loseby, 'Power and Towns in Late Roman Britain and Early Anglo-Saxon England' (pdf)