The UK has an average reading age of NINE years old.
I'm not sure where you got this information from, or if things have changed dramatically in the last 10 years or so, but that's not what this survey published in 2012 says.
https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/36000/12-p168-2011-skills-for-life-survey.pdf
Eighty five per cent of respondents achieved Level 1 or above in literacy, with 15 per cent performing at Entry Level 3 or below.
57 per cent of respondents achieved a Level 2 or above score in literacy
From the glossary: Entry Level 3 is the national school curriculum equivalent for attainment at age 9-11, Level 1 is equivalent to GCSE grades D-G and Level 2 is equivalent to GCSE grades A*-C.
So 57% of adults have literacy levels at or above the level of GCSE A*-C, and only 15% at or below the level of a 9-11 year old child. From this, the average (median) looks to be around GCSE grade C.
(Sorry for the derail.)