You need a free licence from the Swanage Information Centre. Detecting isn't allowed between 9am and 6.30pm - one council document limits this to 1 May-30 Sep, the beach regulations state it with no dates, so assume all year unless the Information Centre tells you otherwise.
"Metal detecting is allowed apart from between 9.00am and 6.30pm each day (1st May and 30th September). A licence is required which can be obtained from the Swanage Information Centre."
Tap any spot in England on the UK Detector map to check it against scheduled monuments, SSSIs, nature reserves, National Trust land and access land in one go. Free: 3 checks a day.