To be honest I think the number of tests the ACU can provide a chain with is still peanuts in comparison to people off the street, so it probably doesn't make commercial sense to do it.
But anyway, one quick Google ("cheap eye test") and the first result was this - a voucher for a £10 eye test at Boots!
http://www.boots.com/en/GBP10-eye-check_1240962/ 