R-1830 was in use by the mid-1930sUntil late 1938 or early 1939 the British are not going to by foreign engines.
They did buy American engines in the Lockheed 14/Hudson.
The first Hudsons showed up with Wright GR-1820-G102A nine- cylinder radials, each rated at 1100 hp for takeoff and 900 hp at 6700 feet., single speed superchargers.
The First Hudson was being test flown on Dec 10th 1938.
Australia had ordered some Hudsons with Twin Wasps. The Version of the twin Wasp they got (with 87 octane fuel) gave 1050hp for take-off and 1000hp at 11,500ft in high gear of the two speed supercharger.
Mercury could give 840hp at 14,000ft. A lot less for take-off.
Designing, ordering and tooling up British factories in 1937-38 for fighters to use American engines that wouldn't exist until 1940-41 seems a bit of a stretch.