Boots Opticians will do it, and they charge £15.
Just let them know that you need your form signed when you book - they shouldn't charge you extra to do this.
Forms can be downloaded off the ACU site:
http://www.acu.org.uk/AboutACU/Publications.htm If your company requires you to use a monitor, they may also pay for your eye tests - worth checking out although at £15 it's not going to break the bank!