Rainy days

As the people of Gloucester recover from the aftermath of the floods, it has to be noted that the area has a history of such problems:

Dr Foster went to Gloucester
In a shower of rain
He stepped in a puddle
Right up to his middle
And never went there again.

In olden days before tarmac and traffic, this rhyme served as a warning to children to avoid jumping into puddles which appeared shallow but could be much deeper.

Dr. Foster is said to refer to King Edward I of England. He travelled to Gloucester in the middle of a rainstorm. When he arrived, his horse fell and both he and the horse ended up in the middle of a huge mud puddle. The townsfolk had to use planks of wood to remove the king and his steed from the mud. King Edward was enraged by this misfortune and refused to return to Gloucester.

So far I haven’t been able to Google an adequate explanation of why King Edward should be called Dr Foster - apart from the obvious fact that Foster rhymes with Gloucester while Edward rhymes with nothing. (although rhymes could be found for the abbreviation versions Ed or Ted).