It's worth reminding ourselves -- in the light of the erratic hunting going on on Flat Holm and Steep Holm -- what the Pleistocene context is, according to other authors. I'll stay out of it for the moment.......
On this map you can pick out Lavernock Point to the south of Cardiff, and you can see the location of Flatholm and Steep Holm in the mouth of the Severn Estuary, where it broadens out into the Bristol Channel proper. Click to enlarge. Note that there is a deep channel (over 20m deep) between the two islands.
There is a complex sea floor here, with a great deal of sediment movement, given the vast tidal range and the fierce currents that sweep in and out twice a day.
You can see from this Landsat image just how stained the water is within the estuary -- sediment is carried well out into the Bristol Channel on every falling tide.
How many times might Flat Holm and Steep Holm have been glaciated? The current consensus (not that we should pay too much attention to it) is that this part of the Severn Estuary has been glaciated just once -- during the Anglian Glaciation (coinciding with Marine Isotope stage 12) -- about 450,000 years ago. This is a reconstruction of the Anglian ice limit by Pawley et al -- and in view of the fact that glacial deposits are now known from well within Somerset this may be taken as a very conservative line drawn on a map: