DEC positions cannot be tied to a particular base. The Master Seniority List determines who goes where, and if there is a demand for people at LGW at the time you are employed, you can go there provided people ahead of you in terms of seniority are not taking those spots. Likewise, if the base is downsized, lowest seniority will be affected first.
The MSL is strictly enforced by the company, so it may take a while for you to get where you want. As far as pay is concerned, get OSM to send you a sample contract. Perks are there, free commuting from home to base, crew meals, uniforms, job related expenses are reimbursed and decent staff travel for you and your family, and even friends.