In the UK mode 'S' is only under trial at specified locations. So it is not possible not to allocate a mode A.
Although there are currently no delays in UK it is only because codes are re-used quite quickly. This does occasionally trip the NAS computer up and aircraft are occassionally issued the same code, although not for long usually. Something that is being worked on. I would not like to speculate as to how close to capacity we are but given the general downturn in traffic not ver very, just quite
