I'd be disappointed if the "Best ever British sporting achievement" was any specific event. Much as the Tour has captured and captivated over the last few weeks, it's only one event. Winning over a series of events or over a longer period (such as Redgrave or Pendleton) are much better achievements, IMO, as was the stuff done by Paula Radcliffe, Rebecca Adlington, Jessica Ennis, Ben Ainslie and a host of cyclists (Hoy, Boardman, Cav etc). Wiggins himself has some pretty special achievements, not just switching from World Champion on track to road racing but winning the Dauphiné and Paris-Nice as well as the TdF this year...
Yeah, it's a huge thing that he has done today but it's just one thing and suggesting it is the best ever by a Brit is, IMO, belittling all those who achieved at a similar level so consistently over a number of years.
Also, as said above, it would be a little bit tragic IMO, if our best ever sporting achievement was to win it once when Armstrong has won it seven times.