Re: the Channel Tunnel, I agree with Legbreaker that a number of factors are going to combine to render the Tunnel unuseable for a very, very long time. I'd also imagine it's likely that the French would seal off their end completely to keep out any spies and / or refugees attempting to enter France from England (presumably a few tons of concrete would do the job?).
There are actually three tunnels - two rail tunnels and a service tunnel, which is a road tunnel. Perhaps a GM who wanted to make the Tunnel semi passable could make use of the road tunnel?
More info here...
http://www.eurotunnel.com/ukcP3Main/...TunnelVehicles