You forgot about the VAT and import duties that Ali include in the price shown for the UK. They aren't included in the US price - they may get a bill for the import duties and/or tariffs on or after arrival, plus state taxes they're meant to pay. Assuming an exchange rate of 0.74 and 20% VAT it should be £382 - I don't know if there are other duties to add, so the gap might close further.
That's the point though when it comes to the Topping Store, it's £431 because they don't do any currency conversion. They geo-lock currency based on the destination shipping address and then take an approach that $1=£1=€1. When it comes to Ali, yes they do charge VAT on baskets under £135 (whether they actually pay it to the UK treasury is a different matter) but beyond £135 the VAT is deducted from the basket and becomes chargeable on arrival, same as any other country. On that basis the price a UK or EU buyer sees when buying direct from China, on a basket value beyond £135, should be the same as their core USD base price, and if they won't allow payment in USD then it should be reflective of the currency conversion from USD at the time of sale, as per the calculation in your comment. It should never be the case that $1=£1=€1, which is just plain old price gouging.
The example here would be that the Topping Mini300 is $119 on Topping Store (interestingly it says "all costs included"), so $119 plus 20% (for EU / UK) VAT is $142, which would be about £105 or €103 today, yet Topping Store locks the price at £119 / €119 based on shipping address, so effectively we are being overcharged £14 as a UK buyer, or €16 as an EU buyer, for the same item shipped from China. In the past this was never a problem, because we just paid Chinese sellers using USD, regardless of location, but this is getting more and more difficult to achieve, and is why this is not about just Topping, but seems more like a national directive in China to deal with the economic slowdown as salaries and standards of living continue to go up (which is a good thing).
Back to the amazing value of this stack:
I just added it all to my basket on Aliexpress, (Mini300 + E50II + L50), and there is also now a £20 discount for paying with paypal, so adding also the £50 discount on order value, the whole stack can be acquired for £283 exc VAT (£339 Inc VAT, should it be charged), which is astonishing value.
Same stack direct from Topping Store with geo-locked pricing is £431 inc VAT.