But why would it need to bother?
Let's suppose the ability of a minister in the Church of England to be a bishop - it requires attributes not all ministers have, no aspersions cast on non-episcopal candidates - is a normal distribution.
And then let's suppose that the selection of bishops is perfect - that the most suitable non-bishop is chosen each time to be consecrated to the next episcopal vacancy. Let's ignore variability between suffragans and diocesan bishops, and the truth that some bishops may or may not fit certain posts better, for reasons of church tradition or whatever.
Then the proportion of men selected up to now as bishops might look something like this. (Bishops in a suitably male colour, like a nice episcopal purple):
But now that women can become bishops, assuming the same things about distribution etc - why would you need positive discrimination? That same graph for female priests, remembering there are fewer full-time priests who are women, would look a bit like this (yes, I know, should be a bit narrower, but whatever). In this case I've coloured the number of women who are as qualified to be bishops as the men who are already bishops, in a nice feminine colour. For contrast, it's a nice episcopal purple.
And sure this is only a bit of fun, as I get fed up with Burton faffing around trying to do this properly on a spreadsheet so I'm hand-waving rather than doing real maths. And I know there's less accuracy in the selection process, and they'll have missed good men who will be caught later. And I know there will be new, young exciting male would-be bishops coming through all the time. But the long and the short of it is, by having 20 years of women being priests but not allowed to be bishops - there's gonna be a whole bunch of women just bashing against the stained-glass ceiling.
In fact, if there is any need to favour women over equally-qualified men, then the logic would say - that means the criteria are still weighed against women. Sheer stats will mean, given a level playing field, that more than half of all new bishops should be women for the next four or five years. After that, it should still be a higher ratio than the current 5:1 ratio of men to women in full-time priest jobs, until all ratios normalise. I'll say it again - if it takes positive discrimination, then the criteria for selection - whether explicit or implicit - are wrong. The C of E doesn't need to do anything to upset the Blimps. Except let the nature of statistics take its