how do literacy rates in the uk compare between 1956 and 2006

It's just got to be worse in 2006, thanks to the increased immigrant
population, illegal and otherwise.

But how can such an unknown be quantified?

It can't have gone down to much, probonopublico-ga. Current or recent
statistcs such as the one in the CIA world-fact-book give the UK a
literacy rate of 99% like it does for many other first world
countries. That is a rate so high that it can't have been much higher
fifty years ago. It is also too high to analyse very much and still
get meaningful results. Statistics such as the ones on
www.literacytrust.org.uk go much more in depth and using them it is
even possible to make reasonable comparisons from year to year and get
anything other than "still almost everyone here can read and write."

Unfortunately there won't be any similar in-depth statistics avaidable
from 50 years ago. One might be able to dig out some decades old
statistics but they will almost certainly be using different
defintions and methodolgies than the official statistics used today.
This is the same problem you get when comparing international rates
since every country uses different definitions for literacy. The best
you will be likely able to get without a lot of work will be some sort
of general trend.

I think one problem is defining literacy. Although there are concerns
about a high percentage of people being "functionally
illiterate/innumerate" perhaps it's only that it's more visible now
that there aren't the "opportunities" to leave school at 14 and go
into a factory.