Like Silkman, I too did some summer work at LHR.
Things now will be much stricter regarding security issues, however if you sort it all now, there shouldnt be any reason to stop you.
Have your references sorted. 5 years COMPLETE history. Also they need proper dates, (03 March 2004 as opposed to March 2004),
Any gaps longer than seven days need ot be veriefied by a personal reference.
The disclosure thing is new and can take a while too. (Just a criminal record check i believe)
If that fails, why not try at a local flying club as ops or airline crewing/ops which may not be based "airside".
Good luck