Which broker is best for long-term investing into IWDA ETF? Planning to do either 1k/mth or consolidate and do it quarterly/ half-yearly.?

IWDA tracks the MSCI World Index, with an expense ratio of 0.2%. You may want to consider SWRD (from SSGA) instead which tracks the same index but at an expense ratio of 0.12%.

Because these funds are listed on the LSE, you can only use Interactive Brokers, Saxo or Standard Chartered. Saxo is too expensive (8 GBP per trade, 0.12% p.a. custody fee), so that leaves us with Interactive Brokers and Standard Chartered.

Interactive Brokers (Tiered pricing) has better forex rates and lower commissions, but charges 10 USD minimum commission monthly. 3 USD if you are below 26.

Standard Chartered has no minimum or custody fees, but has worse forex rates (0.3-0.6% spread) and minimum 10 USD commission per transaction.

If you invest monthly then Interactive Brokers will be the better choice. For less frequent intervals, you would need to do some calculations to compare.

SCB, no custodian fees.