It is not impossible to extend the runway at the eastern end but the amount of infill required was phenomenal, I can't remember the figures now when we looked at it but it would have required almost every dumper truck in the UK. The 'Herts County are anti' argument is a red herring or it was in my day.
Even more problematical was extending twy A keeping it at the same distance from rwy as now. Even more infill required or you would have to bend the twy closer to the rwy which would would mean the Cat II/III holds being where they are now.
Nothing insurmountable, just depends what you want to live with.