According to the Forestry Commission:
Top leaf-peeping walks include:
1. Westonbirt seasonal trail, The National Arboretum in Gloucestershire
2. Symonds Yat Rock to Biblins loop walk, The Wye Valley in the Forest of Dean
3. Radnor waymarked trail, Bolderwood, New Forest, Southampton
4. Lynford seasonal trail, Lynford Arboretum in Thetford Forest, Norfolk
5. Church path trail, Salcey Forest in Northampton
6. Red Walk, Great Wood in Somerset
7. Bedgebury Pinetum and Forest, Kent
8. Grizedale Tarn Trail, Grizedale Forest, Cumbria
9. Blackmore Trail, Delamere Forest, Cheshire
10. White Horse Trail, Friston Forest, East Sussex
You can find a map of Forestry Commission site here.