This is a coachbuilt (think bus or removal van) body built onto what is probably a KB (TK) chassis (supplied to the bodybuilder without a cab).
Such a vehicle frequently had twin headlamps when equipped as a bus or coach (though usually on a larger chassis with a bigger engine).
The 4 cylinder engine was only fitted to 'low end' models of the TK range and was 220 cu ins (3600cc or 3.6 litre).
You should have a chassis plate somewhere (maybe on the passenger side wheelarch or maybe on the door pillar) that relates to one of the following models:-
http://www.avi-ltd.co.uk/id18.html
The string of letters beginning with E is the model designation (which replaced the earlier KA, KB, KC etc) and this will be needed to identify the vehicle for spares. There will be a string of numbers afterwards that identifies the date that your vehicle was built and will be needed to define any changes from earlier versions of similar vehicles.
You should also have a bodybuilder's identification plate (might be Willowbrook or Marsden or a particular builder of horseboxes) that will be needed for body spares (Bedford doors and windscreens will not fit).
I have found a photograph of your actual vehicle (the same photograph that you quoted) but it doesn't provide any further identification as to the name of the bodybuilder - but you should have that information either on the plate or maybe on the registration documentation (or the badge on the front!). The text claiming 7.5 tons GVW suggests a KC model, but the wheelbase looks short for a KC, so it might be a KB fitted with KC suspension to uprate it to 7500kg GVW.
As far as I can remember the hand warning lamp means
Stop (ie don't drive) as there is a fault with the (usually) braking system.