I'm not suprised you couldn't find anything on Google, the 'modern British coal industry' consists of about three pits and some opencast. Most of the stuff in your HHAs will be imported, on its way from a dock to a power station.

- Woolley 1983ish cropped.jpg (108.1 KiB) Viewed 2084 times
This was West Side Washery at Woolley near Barnsley in about 1983. It's a rotten pic, sorry, but it was taken from a long way up with an instamatic camera. It was brand new then, and state of the art. The rapid loading bunkers are the two grey towers in the top right hand corner, the large dark grey structure is the washery itself. Lots of concrete and dark grey box profile steel and it's
huge !
If your HHAs were being loaded at a bunker in this country it would look something like this. Shortly afterwards I was lucky enough to get a tour of Westoe colliery in the north-east. The loading point there was a massive concrete pad with 4-wheel bucket loaders loading HAAs. No bunkers or other expensive infrastructure, just coal stockpile on one side and MGR train on the other. (I presume there was a conveyor there somewhere bring coal to the loading point but I can't remember). You might be better with something like that, grey cardboard for the concrete pad and a couple of Knightwing Portakabins.
Edit: If this works, this is the loading point at Chalmerston opencast in Ayrshire - just a concrete pad with a conveyor and stockpile. You can even see the yellow bucket loader trundling about.
http://maps.google.co.uk/maps?f=q&source=s_q&hl=en&geocode=&q=waterside&sll=55.423558,-4.564819&sspn=0.428644,1.231842&ie=UTF8&hq=&hnear=Waterside,+East+Ayrshire,+United+Kingdom&ll=55.337624,-4.41586&spn=0.001678,0.004812&t=h&z=18