What's unique about this collection from TheGenealogist is easier searching for those transmigrating through England in order to then board one of the large transatlantic passenger ships. The preliminary journey has been recorded for many transmigrant passengers within the BT27 records. For the first time these can be easily found using a unique transmigration button. The system also attempts to recognise groups of people on the same voyage as a potential family
While the whole BT27 collection from the (UK) National Archives under the title Board of Trade: Commercial and Statistical Department and successors: Outwards Passenger Lists spanning 1890 to 1960 is available from findmypast.co.uk. It's always an advantage to have another source with different search capabilities.