The delightful gardens at the Old Rectory in Farnborough, Berkshire are open under the NGS. John Betjeman once lived here.
Alan Gray and Graham Robeson designed this lovely garden at East Ruston Old Vicarage without any assistance.
Isle of Wight,
Osborne House and Gardens once the retreat of Queen Victoria and Prince Albert
Osterley Parkl is a spectacular mansion open to the public with fine Gardens.
Overbecks Gardens are well worth seeing.
Oxfords best kept secret - The University of Oxford Botanic Garden probably has the most compact yet diverse collection of plants in the World