Complex sentence: Much of the undeveloped part of this area lies within the designated Metropolitan Green Belt, which covers nearly all of Surrey, eastern Berkshire, southern Buckinghamshire, southern and mid Hertfordshire, southern Bedfordshire, south-west Essex, and western Kent.
Split into simpler sentences: Much of the undeveloped part of this area lies within the designated Metropolitan Green Belt. The Green belt currently covers nearly all of Surrey, eastern Berkshire, southern Buckinghamshire, southern and mid Hertfordshire, southern Bedfordshire, south-west Essex, and western Kent.