Random, but I just noticed this: the bridges over the M4 approaching to the Prince of Wales Bridge appear to be encased with... fiberglass panels? or some other nonstructural material. The ones carrying the M4 are simple concrete beams for the short spans and steel plate girders for the longer ones. If they're in fact encased, is there a reason other than aesthetics? Britain is fully capable of building similar-looking structures out of exposed concrete or steel. And what's under the encasements?
I've tried to find an answer for you, not had much luck - but it is probably just an aesthetics thing. In the 1990s we had a sudden heel-turn about what motorways should look like and a few experiments were made with the few lengths that opened that decade. Locally to me on the M65 (the final section finished in 1997) all the bridges used steel I beams and concrete - the beams were all painted blue.
The M4 and M48 over the Severn are considered some of the highlights of the entire network, so no surprise some effort was spent on the bridges - normally cost cutting means we go for the most basic design going.