I used to work for an environemtnal consultancy who had a small GIS team. The GIS guys generally came from either a database (SQL, etc) background or from web design. Some of the younger members of the team had qualifications specifically related to GIS but most just had a very thorough background in programming.
I am interested that you say you want to get into soil and air contamination work, and that you would be interested in getting out of the office sometimes. I would note that, in my experience, a pure GIS job involves sitting in front of a PC. If you want to get out and get experience in contaminated land investigation, then you might be better making an informal enquiry with some of the larger consultancies with environmental groups (Atkins, Peter Brett Associates, Black&Veatch, URS, Enviros, etc.) about a junior position in EIA and contaminated land investigation with the possibility of working on GIS.
So you know, I started (with an MSc) on about £17,250 and after five years was at project management level doing technical modelling, earning around £30k. There wasn't much of a London weighting as far as I can remember. I think the most I would have earned without going into people management would have been about £40k. The EA will pay less than a consultancy, but you get significantly better T&Cs including flexi and way mor holidays, which more than makes up for the difference in pay. However, from my recollection, most of the GIS work (at least for the NW) was contracted out, rather than done in-house by the EA.
You could also look at digital mapping providers like Landmark and BlueSky.