I agree its very unlikely, for a number of reasons, not least of which, can you trust the UK caravan manufacturers to assemble a standard product correctly? Yet alone products which deviate from a standard product, or even "Customer Specials"
From what I understand of most UK caravan construction, they haven't got the systems in place that will allow them to link a specific customer order to the first stage of production (i.e. the chassis or chassis number).
But the idea of having optional significant mechanical upgrades available affecting statutory criteria posses not only logistical issues for the manufacturers, but within the present type approval processes, each specific variety would have to be separately "Type Approved" approved before they could legally sell it.