Agree with Mears that in the NHS trust that I work for, there are no midwives on call for a situation like you describe.
A midwife from the birth centre or delivery suite will attend an unexpected delivery at home or an imminent delivery, but would not attend to care for someone who phoned up in labour asking for a homebirth. There simply are not the available staff to do this, and there are no bank or on call staff waiting for a phone call at 3am who would be able to come in for this, never mind having the experience in homebirthing or the confidence to do it.
Anyone in our area who wants to book a homebirth is referred to a small team of midwives who caseload mainly very young or vulnerable women (and homebirthers!) but they are not on call except for their own clients, so cant be called upon to attend homebirths, and our community midwives do not do homebirths (no 'out of hours' cover in the community).
So in the case of the central London trust I work for, you would not even get an ambulance sent, but would be given the advice that you should come in to labour ward. If the birth is imminent, and you phoned up again and could not make it to hospital, then a midwife would be sent out, but you would not be getting the good experience of a planned homebirth, instead you would most likely get a frightened and stressed midwife who has probably never even seen a homebirth. Im sure thats not what you would want.
Obviously I dont know your situation, Asur but if you really want a homebirth (which is an amazing, magical experience), then I really suggest you book one with your local hospital instead of risking disharmony and unpleasantness while you are in labour.