Try also looking at the handling agent websites directly.
www.servisair.com have got the "join us" section where they advertise jobs. At the moment there are no LHR jobs just LGW and STN.
www.aviance.com have also got a section where to send the cv.
www.britishairwaysjobs.com and I've heard the Virgin Atlantic are recruiting for the summer up to the 9 of February.