I agree with you on the likelihood (or rather the UN-likelihood) of large predators coming through the tunnel. Not in any numbers sufficient for a breeding population at any rate.
However I fail to see how a scant 22 miles of the Channel is any barrier to rabid bats.
For most European bat species, 22 miles is a very long way to fly, and many, if not most, will not fly away from their preferred habitat. A large expanse of water like the channel is a highly effective barrier against most bats. Some will cross, inevitably, but it has to be enough to start a viable population. Let's just say it hasn't happened yet.