If your bank does not have a direct relationship with DMA's bank (Crown Agents Bank (UK) [CAB]), your bank will need an intermediary/correspondent bank to facilitate transfers, a service for which the intermediary/correspondent bank will charge. The intermediary/correspondent bank then acts on behalf of the beneficiary bank (your bank).
The intermediary bank/correspondent bank is the bank that receives the bank transfer for your bank in the country of the sender (UK). Example: If you want to draw money from your trading account to your bank account in South Africa (FNB/ABSA/Standard Bank etc.), and there is no direct relationship between CAB and your bank, then the money is first transferred to the intermediary/correspondent bank for onward transfer to your bank and subsequent deposit into your account.
DMA transacts on a "shared costs" basis for withdrawals out of your trading account – meaning the charges are split between DMA and yourself. DMA will pay its bank charges and any other payment order costs, while you will bear the charges levied by any other banks (intermediary/correspondent, beneficiary bank etc.) and you will receive the balance.