just remember that 3/4 of the total circumferential band of the Tyre can be devoid of tread and still meet the pass requirements .(official M.O.T. statement).. in other words, you're Tyre can be bald as a coot on the outer edges,( no canvas or rips) but still be legal if 3/4 of the middle of the Tyre are on or above the tread depth. 1.6mm.
so the next time you have a look at your Tyre,s and the edges are bald. seek proper advice on their condition, not a Tyre sales person . cos they will just sell you new ones .
now thats way off topic ..