The old islands show up well on LIDAR surveys, which capture small differences in elevation. These are now available on the NLS site
https://maps.nls.uk/index.html
This is part of Cambridgeshire side by side with an OS map -
I would suggest looking around the green areas.
As an aside, the scottish LIDAR surveys have only recently been added and poring over these recently inspired my Dere Street entry. The Scottish part of the survey has been processed to give shading from three different directions in blue, red and yellow, which helps pick out small features. The English one above is only shaded in one direction in green.
Below is a zoomed in part of Dere Street a couple of miles south of my entry, where it is only a faint track across moorland. Not only does the road show up well, but the small quarry pits at the side of the road from where the stone and gravel used to build it was dug still show up clearly after nearly 2000 years.