Is that the high volume of EHCP students which you have actually confirmed is the average number? Appeal places are not 'less available in good schools because of the high number of EHCP students.' That's nonsensical. Unless you're going to say that they are less available due to the LAC, the siblings, the children who live nearest the school, the children who qualify for a place because they attend a feeder school... Where do you draw the line?
Every school admits up to their PAN. Some schools, either in agreement with the LA or despite the LA, go above their PAN. At that point they are (nominally) 'full'. Appeals panels have the job of testing whether schools are actually full, and then, if they are, testing whether they are so full that any appellants should be denied a place. Then, if they determine the school is not so full as to deny all appellants a place, they determine which appellants have the best case for taking the space that has been determined to be available.
How the school got to the point of being nominally full makes no difference to the appeal process. If there wasn't an EHCP student taking the place, there would be the student at 1.7 miles instead of 1.5 miles, and there would still be appellants who lived 1.8 miles away who just missed out.
Added to all of this, some of those EHCP places will go to applicants who would have got in as a main round applicant anyway. For example, in my small town, there is only one secondary school which is allocated. Unless you have an EHCP, you don't get a choice. Public transport isn't great, so even with an EHCP, it's unlikely that a parent would choose the next nearest schools because, largely, the LA would expect them to transport their child because there was a closer suitable school. Therefore, if there are 35 EHCP holders, it's likely that at least 33 of them would have gone there shush l anyway.