The majority of the following post is what I think, not what I know so I might be wrong.
The general engineering degree at Heriot Watt can cover all the engineering disciplines (mechanical, electrical, civil ext) expect from chemical. This is because of the level of chemistry covered in 1st and 2nd year of chemical engineering. In 1st year, chemical engineering students do the standard theory modules that are core for a normal chemistry degree and then a few others that are relevant in later years. So to answer your question, no I don't think that by doing H100 you can get into chemical engineering with that route. You would probably be out of your depth in the amount of chemistry they expect you to know.
It's still worth sending someone on the contact info bit an email to double check on this.