Thanks for your reply.
A right of way (ROW) "attaches to the land" it is granted.- ie that land (your land) gains the right of way, as opposed to the person who owns the land or for a particular purpose, unless the ROW is restricted in its wording.
So, if the ROW had said "I grant a ROW to Tonard Limited for the purposes only of them accessing their workshop" or worda to that effect, then that ROW is only to be used for that purpose.
However, it appears from what you tell me that you have a general ROW for the benefit of your land. Therefore, the right granted is to allow you access to your LAND, and therefore whether your workshop remains in place or not, makes no difference whatsoever.
I am therefore of the view that your ROW will remain if you were to demolish your workshop.
I hope this assists and sets out the legal position to you.
If I have helped, I would be grateful if you could rate my answer.