Custodians of Scheduled Monuments know that Scheduled Monument Consent (SMC) is required for most works and other activities that physically affect a scheduled monument. In practice this is a very strict regime under which very little, if any, disturbance of the monument is possible without consent, apart from activities covered by Class Consent and which are most commonly relied upon are those relating to agriculture, horticulture (gardening) and works urgently needed in the interests of safety and health.
SMC is separate from the statutory planning process. However, the two processes can run in parallel if planning permission is also required for proposed works to a Scheduled Monument - (Source). It is analogous to Listed Building Consent, it is an additional consent needed not a replacement to the normal planning permission requirement.
Planning permission is also needed if the work being carried out meets the statutory definition of ‘development’ which is set out in section 55 of the Town and Country Planning Act 1990. In this Act, except where the context otherwise requires, “development,” means the carrying out of building, engineering, mining or other operations in, on, over or under land, or the making of any material change in the use of any buildings or other land.
The categories of work that do not amount to ‘development’ are set out in section 55(2) of the Town and Country Planning Act 1990. These include building operations which do not materially affect the external appearance of a building. The term ‘materially affect’ has no statutory definition, but is linked to the significance of the change which is made to a building’s external appearance.
It seems that obtaining an SMC has been too often an excuse to not to also obtain Planning Permission, and to avoid the democratic overview that the planning system provides and the obtaining an SMC doesn't. Planning permission approval also provides environmental protections and a range of conditions can be applied.
If any cases come to mind now or in the future remember the appropriate remedies are available.
Remember this is not legal advice - I'm not a lawyer and it is worth what you paid for it, nothing. It is just my opinion.