If no building currently exists the fact that one was there in the past may not help you get PP for a new one directly. However in some cases it helps establish the use class of the land as industrial rather than say agricultural.

Not sure if this is still current or if it applies to a national park but...

http://www.planning-applications.co.uk/agric.htm

Section 55 of the Town & Country Planning Acts 1990 provides that:

"the use of land for the purpose of agriculture or forestry (including afforestation) and the use for any of those purposes of any building occupied together with the land so used" does not constitute "development".

So you might not even need planning permission? In which case you might want to look at a Lawful Development Certificate rather than a planning application.

http://www.planningportal.gov.uk/permission/responsibilities/planningpermission/ldc

http://www.communities.gov.uk/documents/planningandbuilding/pdf/developmentcertificates.pdf

What kind of workshop? Will it be used to support the forrestry activity (eg storing and maintaining tools) or as some sort of educational training workshop? That might make a difference to how you approach the planners if PP is required.

Even if it does still apply there can be other rules that mean planning permission is required.