As a day of reckoning plays out in across global markets, some companies listed in the FTSE 100 (^FTSE) will be glad they hedged their bets in the US ahead of US president Donald Trump's sweeping tariff decisions.

As widespread so-called "reciprocal tariffs" were brought in by Trump on Wednesday, many companies doing business with the US will be thinking of how to restructure deals and the products they produce.

The UK received a comparably more lenient 10% levy, while EU countries face a 20% import tax to the US. Meanwhile, China was slapped with a 34% tariff which it said on Friday it would reciprocate.

As countries scramble to renegotiate their terms with one of the world's biggest trading nations, companies are right to be worried.

“UK companies selling into the US could see a slowdown in sales if US customers deem goods too expensive under the new tariff regime," said Dan Coatsworth, investment analyst at AJ Bell.

Read more: How Trump's tariffs will impact your finances and the UK economy

"It means companies need to assess all their options to keep their engines ticking over smoothly and not spluttering."

He added: “One way for UK companies to get around tariffs is to build more products in the US. Investing in factories or offices creates jobs for Americans and could bring significant investment to industrial heartlands, something that would please Trump no end."

AJ Bell looked into the FTSE (^FTSE)-listed companies with the biggest proportion of their operations in the US.

Some UK companies have already planted flags Stateside. There are 20 companies in the FTSE 100 (^FTSE) index with more than 20% of their facilities in the US, according to AJ Bell analysis of Bloomberg data.

This suggests that markets might have over-exaggerated the tariff hit to certain stocks. If a big chunk of goods are made and sold on US soil by UK companies, there are no tariffs to pay.

International equipment hire company Ashtead (AHT.L) topped the charts, with 90% of its facilities located in the US.

Meanwhile food service company Compass (CPG.L) clocked 68% of its sales across the pond.

Credit ratings agency Experian (EXPN.L) also collects just under half of its sales from US markets, with 59% of its facilities based in the US.

Here's the data:

As defence has become increasingly important in recent months, it's notable that BAE Systems' (BA.L) operations are mostly Stateside. Calculations show 59% of its facilities are in the US, with the manufacture of armoured vehicles, ship repair and explosive materials geared to supply the US Department of Defense.

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Meanwhile, approximately two thirds of aerospace tech firm Melrose’s (MRO.L) sales come from America and 29% of its facilities are in the country. Last December Melrose-owned GKN Aerospace opened a $55m repair facility for aero-engine components in San Diego, which was a major financial commitment. It doubles the company’s maintenance, repair and overhaul capacity in the region, AJ Bell added.

“The fact many UK companies are already in the States is important on another level. Having an existing presence makes it easier to press the button on adding more, rather than starting from scratch in the country," said Coatsworth.

By 3.30pm on Friday in London, the FTSE 100 (^FTSE) was down around 4.7%, with all component parts of the index in decline, according to Yahoo data.

The top fallers were mining companies, which are typically vulnerable to trade disputes, and financial services firms and banking stocks.

Read more:

What Trump's tariffs could mean for UK mortgage rates Will Trump's tariffs change Bank of England's interest rate plan? How and when Trump's tariffs could impact UK inflationDownload the Yahoo Finance app, available for Apple and Android.

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