If someone forges a, let us say, a French licence and uses that in the UK, what chance do the authorities in the UK have if there is no exchange of data. The whole idea of a European licence is to make that possible.
I can only describe the decision to allow 2.5m wide caravans only behind vehicles with a GVW of at least 3.5 tonnes to be amateurish and certainly shows no signs of wisdom of whoever dreamed it up. I am sure there can be no data to support the need for such a restriction.
I really don't now how warning triangles, reflective jackets, etc. come into the picture because that has absolutely nothing to do with the EU (other than the design of the reflective jackets is according to European standard, so that one common type is acceptable in all countries where they are required).
Speed limits are and always have been picked to suit local traffic conditions, so there is absolutely no need for common European-wide speed limits.
As far as caravan width is concerned, I see no conditions which are so specific to the UK that make it so important to go it alone. I can legally tour the UK with my foreign-registered car and a 2.5m wide caravan without as problem, so why should I be privileged?