Sorry but there is already a pedestrian tunnel under Townwall Street - it is, despite the rather excellent nautical tiles, dark dingy and seriously uninviting.
What we need is the land bridge that the DHB talked about some time ago ) something similar to this
http://www.sublimephotography.co.uk/eastendphotos/milendpk/pages/greenbrpan.htm would be wonderful) connecting the sea front via the Wellington Dock to the Town by the scrub land at the bottom of York Street.
I am not sure why you would want to put a tunnel, even if it was possible, under London Road, a better idea would be to put light controlled crossings in and reduce the speed limit to 20mph from the Town Hall all the way up to Coombe Valley Road.
I do agree that town centre traffic needs to be reviewed and consideration given to restrictions on who can go where when.