There is a suggestion that the glacial gradient from South Wales to Stonehenge was sufficient for ice to flow along it:
Just a reminder that during the last ice age the Bristol Channel was above sea level and was a deep wide valley. Glaciers flow downhill so rather than the ice floating up the channel and landing on the Somerset coast from south Pembrokeshire it would have followed the gradient.
Figure based on: "Location and bathymetry of the Bristol Channel. Contours are depths in metres relative to mean sea level" from "Impact of Tidal Energy Converter (TEC) Array Operation on Sediment Dynamics" - Scientific Figure on ResearchGate. Available from: https://www.researchgate.net/figure/Location-and-bathymetry-of-the-Bristol-Channel-Contours-are-depths-in-metres-relative-to_fig4_260052339