Why not? The Environment Agency are expecting this tide to be 5.67 metres (18.6 feet) above ordnance datum (that's mean sea level not low tide) so it seams reasonable. Alert details here.
Because mean sea level is like you say, an average. We are currently on spings so they will be higher than neap tides but even so nothing exceptional, hence why matthew is on his way to chester shopping not rescuing his house from the sea!