I think if you can fit one inside the other whilst passing through security (and not breaching the maximum dimensions of the single item of hand luggage), then I think you should (mostly but not definitely) be fine and can get it out again once you've passed security.
Just to clarify that my uncertainty results from, I believe, someone mentioning that BAA (or BA ?) insisted on the one bag rule at the gate (which doesn't make sense given that you can buy any number of bags of duty free and - so we're told - take them on board). However, if that is a probelm, then put your bag inside the other again and take it out once you've boarded.
You can also attempt to sneak by more by (e.g.) (i) taking out your laptop at security and then putting stuff into your one bag (where your laptop went); and (ii) wearing and/or putting things in pockets immediately prior to passing through security.