Going back to the OP ...
Does anyone know why we are out of the EU. Why can't ebikes do the same max speed of 20mph as American ebikes do , we are trying to get a trade deal with America that would be one less obstacle.
What happens now is that US domiciled companies like Spesh and Trek manufacture goods in the far east and ship them from there into various target markets around the world, all of which have different regulations.
Due to this market variability they need to do two things. 1) Manufacture market specific variants of products (set the speed limit, include an appropriate plug, and so on) and 2) add market specific packaging (local language packaging, market specific manuals and warning stickers for batteries and so on). They try to do this as efficiently as possible to make goods interchangeable between markets.
They'll have a product compliance team working on this and will try to push as much of this back to the far eastern manufacturing plant as possible with as little rework as possible further down the chain, but they are ready and able to deal with various different markets. It's their job.
Now, how would changing the UK regulations, currently aligned with the EU regs (25km/h), to align them with the US regs(20mph) help at all? It wouldn't make a damn bit of difference. It would just make importing into the European area slightly more of a headache (because the UK would require different product variants), but not much.
And what impact would this have on a US-UK trade deal? None at all.
Of course the supply chain will be more complex than that, for example Wikipedia tells me that Trek do some manufacturing in Germany (but none in the US). But primarily the movement of goods will be from Taiwan and other far eastern locations with some finishing/rework in the target markets. Very little of this is going via the US, which is just another import area.