As an employee of Servisair, I have to say that we regularly come into flak from other members of the aviation community for appearing as an unprofessional, motley assortment of staff, however I must stress as with anything there are a number of employees at the company who are professional & passionate about their jobs, and the comments made by bigmuk is of huge disrespect to the hardworking staff of the company. I will of course admit that at most bases the company is chronically understaffed, staff work long & difficult weeks and the "management" of the company has a lot to be desired, but it's fair to say this is true of many companies, not just handling agents or companies in the aviation industry.
Servisair is a fantastic starting point for anyone wanting to get into the airline industry, providing you with a well-rounded view of all that goes on at the airport. My understanding is that work experience placements only go to those in their final year of GCSE or those who are taking A-Level/VCE studies, preferably those taking either a qualification in travel/tourism or those who have a sufficient interest in it. Most airports simply will not provide any form of temporary airside passes to those 16 or under, so your age is really the only hurdle I can see LGW15. However, do call Servisair at LGW, and no doubt someone can tell you their specific base policy regarding placements. Good luck!