The US is already the UK's biggest export market, accounting for 25% of British exports. 80% of that is machinery and transport equipment, manufactured goods and chemicals and related goods.
The export market from the UK to the US is likely to be hit by Trump's aim to repatriate manufacturing and jobs. American companies manufacture in Europe (to avoid American taxes) then import to the US. This is especially true of chemicals and related goods (ie pharmaceuticals).
Any trade deal between the US and the UK is likely to favour America. We would become just another market for American healthcare, including providers and insurance schemes. We currently have a trade surplus with America, but not for much longer, if Trump has his way. This is especially worrying for the UK in the context of leaving the EU.
We have to import - oil and food probably being the most important - but finding new export markets is going to be tough, unless we become a sweatshop and/or tax haven. The UK has survived on trading and providing financial services for at least 300 years, because we don't actually have many raw materials, so the future doesn't look too rosy.