ATC (Aviation Tooling Company to give them the name of their owners) took over the facilities of the old Dan-Air engineering at Lasham Airfield after BA bought the company and sold the engineering side to FLS, who promptly closed most of it down, as I recall.
Addition of the Southend site happened some years later after the collapse of Heavylift - ATC then acquired that facility and its people.
A lot of water has gone under the bridge since both those acquisitions but certainly when I knew them they both had a good reputation. They seem to have a stable customer base so others evidently share those views.
If you want an apprenticeship and can get one that's 99% of the battle, as the biggest hurdle most people face getting into licensed engineer training is the hands-on aspect. I'd certainly say if you are offered you should grab it with both hands, just don't expect to get rich!