Gatwick to the nearest point on the French coast is only 70 nautical miles, so as somebody above said so succinctly; there is something in the way.
You can determine your radio horizon using the web site www.heywhatsthat.com, there is plenty of info on this site that has been posted by abcd on how to use it.
Note that it uses the physical topography and doesn’t allow for obstructions such as buildings and trees. My theoretical coverage in one direction is beyond 250nm, however actual range is nearer 75-100 purely due to large and dense non-deciduous trees.
The English Channel may potentially affect signals due to variations in the lower atmosphere, and that wouldn’t matter if it were saline or fresh water, but it won’t be a barrier to receiving signals.