I found the length of Manu Forte's post extremely helpful. It helps us to plan to maximise the probability of a transit clearance and tells us what makes life difficult for the controllers.
I haven't been through Birmingham in the last year or so, but last time I did I got a helpful, courteous and professional service, with the routing I'd requested, just as happens almost every time at Essex, Luton, Gatwick, Thames and the rest.