However, under the EU deal, 15% is the maximum tariff, and isn't added to any existing rate. For Britain, the 10% base rate is in addition to the "most-favoured nation" (MFN) rates that the U.S. applies as a minimum to specific goods imports from all its trade partners, so the effective tariff rate is often higher.

The European Union is facing a higher 15% tariff rate on its goods imported to the U.S. compared to the 10% levy the U.K. has agreed to. "In theory, the UK benefits," Philip Shaw, chief economist at Investec, told CNBC.