The derived SI unit for bad English accents is the Van Dyke and was introduced in 1981 when it became clear it was required in the film casting industry. Its definition in terms of base units is complex, but it was constructed such that 1 vD should be equivalent to an objective measurement of the accent of Dick Van Dyke in the movie Mary Poppins ("Gor bloimey Mairry Poppins!").
For all practical purposes 1 vD is vastly too large for general usage. More usually, English Accent Awfulness (EAA) is measured in milli Van Dykes ("Paltrow got her EAA down to 6 mvD in that take") or micro van Dykes ("Zellweger is easily in the micro vD range").
Unconfirmed reports do suggest of localised readings of up to 5 GvD at the Texas RenFaire.