It varies depending on PoD.
With earlier changes I'm very prone to preserving the older shires (e.g. Hallamshire, Pecsaet/Peakset, Hwicce/Wychset), rather than using the ones imposed unilaterally by Wessex following the end of the Danelaw (the "traditional English counties"). I favour Mercia over the other Old English kingdoms, by virtue of having been born and grown up in Birmingham.
With later PoDs, I tend to introduce Provinces, as sometimes mooted during the repeated 19th century debates about electoral and administrative reform, making Yorkshire, East Mercia and West Mercia each into unified provinces for example.
Aside from fiddling with the Welsh border I tend to leave the borders alone, as I've not researched them enough. (I've tended to spend more time trying to move the Canadian border southwards, and the German borders outwards at the expense of France

)