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HO scale, take a look at the websites of Gaugemaster https://www.gaugemasterretail.com/ and Rails of Sheffield https://railsofsheffield.com/ both of whom stock fairly limited quantities of some of the US HO scale manufacturers (mostly Athearn in the case of RoS and Walthers in the case of Gaugemaster).
Unfortunately UK stockists of US HO are now virtually non existent otherwise bar the second hand market (mostly eBay) and a few dusty boxes of some used model dealers tables at shows. Those like GM and RoS also only tend to stock a very basic selection of models, usually just one or two of each release and then only the full spec sound and features rather than the more sensibly priced vanilla versions.
As for detail parts, forget it. The last retailer of those in the UK packed their bags long ago so importing from the other side of the Atlantic is the only option, if using a US based retailer expect to have to take out a mortgage for the postage ($20+ just to post a single sheet of waterslide transfers for example) however using a Canada based supplier such as TMR will be much cheaper on the postage and may be worth taking a look at.
GM do stock Piko models but in terms of US outline, that is basically just the KM ML4000 recently released, and they only had two or three of each version there too.,