The airspace around BHX is quite complex and there are many reasons why a straight in will not be allowed. As a pilot I may not be best qualified to answer your query but I think I have got a good grasp of what goes on.
As far as I am aware the handoffs between London and Manchester to Birmingham are controlled by 'Standing Agreements' which cut down on the coordination e.g. always being handed off at FL80 to HON. The Manchester and London frequencies that deal with aircraft just before they are handed off are both complicated and often congested at times since they deal with all sorts of traffic and I am sure they would not have a great deal of time to phone Birmingham to ask if a straight in approach will be likely for every arrival. The chap on the Welin (130.925) sector at London doesn't know off the top of his head how many Birmingham inbounds the Stafa (134.425) controller at Manch is controlling and vice versa. So at best they will have a rough idea if you will get a straight in or not!
Remember that if you are going to HON at FL80 you could be the only inbound in the sky but remember the southbound SIDs off 33 which turn you to the east of the field. So it may be quiet with inbounds and you may still be kept high since the departure off 33 could cause a confliction. The chap on Welin (130.925) will probably have no scooby if this is likely to arise unless he phones Birmingham which is why sometimes it may seem they guess it will be a straight in for 33 and it turns out to be vectors after HON!
Are CDAs in use now? Are the airport publishing performance tables?