The driving test requires that you are able to read a license plate at 20 metres (22 yards).
Regardless of any test that may or may not be brought it it is actually an endorsable offense (3 points) now to be driving with eyesight below that.
There were some spot checks in Wales recently - only 5 of the 300 failed but that's still 1.5% of the population who are driving about unable to meet a very low standard of vision.
http://www.worldhighways.com/news/article.cfm? recordID=9765
There have been calls for a compulsory test at 70 for a number of years it's probably a matter of when rather than if.
If you think you may be unable to meet the standard I'd go and see an optician if I were you - if you can't meet the standard with corrected vision you may have to seriously ask yourself whether it's responsible to continue driving