A. Spinal cord compression: This is the most likely diagnosis given the patient's history of metastatic prostate cancer, which is known to metastasize to the spine, and the gradual worsening of back pain followed by difficulty walking and urinary difficulties. The clinical findings of reduced sensation below the umbilicus, bilateral lower limb weakness (power 4/5), brisk reflexes, and upgoing plantar reflexes are consistent with spinal cord compression.

D. Cauda equina syndrome: Although less likely than spinal cord compression, cauda equina syndrome could explain the patient's symptoms, especially the urinary difficulties and lower limb weakness. However, the presence of upgoing plantar reflexes and sensation reduced below the umbilicus might lean more towards a higher spinal lesion.
E. Nerve root compression: This could be a consideration, especially given the patient's history of previous fractures and metastatic disease, which could lead to vertebral body collapse or metastatic lesions compressing nerve roots. However, the combination of symptoms and signs suggests a more central process like spinal cord compression.

F. Space-occupying lesion in the brain: Although the primary symptoms suggest a spinal issue, a space-occupying lesion in the brain (e.g., a growing meningioma) could potentially cause increased intracranial pressure leading to brainstem compression or hydrocephalus, which might result in some similar symptoms. Given the patient's history of asymptomatic cranial meningioma, this is a critical diagnosis not to miss.
B. Normal pressure hydrocephalus (NPH): NPH can present with gait disturbance, urinary incontinence, and cognitive decline. While the patient's symptoms of difficulty walking and urinary difficulties could fit, the absence of mentioned cognitive decline and the presence of a clear sensory level make this less likely.

C. Radiculopathy: While radiculopathy could explain some of the patient's symptoms like pain and weakness, it typically does not cause the combination of bilateral lower limb weakness, brisk reflexes, upgoing plantar reflexes, and a clear sensory level below the umbilicus as seen in this patient.
Other rare diagnoses might include spinal infections (e.g., abscesses) or inflammatory conditions (e.g., transverse myelitis), but these would be less likely given the patient's history and the gradual progression of symptoms without mention of fever, weight loss, or other systemic symptoms.