How could we get the Halifax Gibbet, a sort of proto-guillotine, adopted for capital punishment across England? I'm thinking that we need to have a revolution and have hanging be associated with the old regime.
ECW, somehow?Really what you'd need is for the Halifax Gibbet to be seen as the more humane alternative to hanging. This was what fueled most of the reforms around capital punishment methods in the 18th and 19th centuries. It shouldn't be too hard - simply have a series of high profile botches with the short-drop hanging process (as the more "humane" long-drop wasn't really adopted until the 1860s) that either weaken people's faith in its ability to work properly or have people see it as simply too horrible.
The problem is, after the 1790s Reign of Terror in France you'd have a hard time introducing the guillotine in Britain as it came to be a symbol of Revolutionary violence and excess. Thus, really, your POD needs to be before 1789.
ECW, somehow?