Speen was designated as a Conservation Area in 1989 by Buckinghamshire County Council. The conservation area boundaries were altered in 1994 by Wycombe District Council. Government
Guidance states that conservation areas should have an up-to-date appraisal. More information Here

Trees in conservation areas:

Before carrying out any work on trees in a conservation area, unauthorised works to trees in a Conservation Area is a criminal offence under Section 211 of the Town and Country Planning Act 1990 (as amended).

This is the link to Buckinghamshire Council’s  section on Trees and hedges guidances