Can a roundabout be classed as (part of) a motorway
Technically and legally, yes. (
Example 1,
Example 2). It's very rare though, requires it to only be involving motorways, for instance. Our one is not.
A good way to think about this is whether it has walking pathways; what's the background colour of the signs; could you legally cycle on here*, etc.
*While you can cycle on this roundabout and the A41, I would never
ever ever recommend it.
So, if you walk a bit down the slip road (and past the 'motorway end' sign), even being there, let alone crossing the road, would be a criminal offence (excepting emergencies or exceptional circumstances).
Regarding pedestrians on 'regular' roundabouts, much as I loathe M5J1, they're free to attempt to cross any part of this just as they could the main road itself - the same sets of regulations apply. Drivers, given the opportunity, have to yield to the more vulnerable user.
There's no criminal offence for any party here, and so we go towards civil law and proof 'on the balance of probabilities'. While normally the driver usually has less to lose and a higher blame, people crossing here really do start to lose the argument to defend what they're doing as safe...
However, this is a product of having an attitude of 'common sense' and is also the reason we don't have any nanny-state jaywalking laws unlike, for instance, the USA. Right of way means nowt when you're flat as a pancake.
Though, in my view, it really should be the council (or whoever is in charge of that road) to definitively sort this one out one way or another before they eventually become liable for incidents due to multiple occurrences...