Haha people don't even read the terms and conditions of the e-visa website. No where do they mention an "appeal" process for rejected or cancelled applications.
I got mine on workcation basis too and actually spend little time in Thailand as I have other stuff to take care of in other countries, so I'm always coming in and exiting. So far no checks are being done (at least not on me) even though I'm using a weak 3rd world passport. You being British should be fine.
Why did you feel the need to use a "legal firm in Thailand" for the DTV? The requirement is that the applicant does it his/herself from OUTSIDE Thailand. This is why Thai visa agents hate the DTV: it's because they can't get a cut out of it. But here you go.
Don't over think it, just reapply somewhere else and keep it moving. Mine was cancelled without reason and I gave zero Fs. Just reapplied elsewhere after a month cooling period and was approved.