In reply to Robin Woodward:
Good advice above. For a boot fitter, I strongly recommend your first port of call be Colin at Solutions for Feet in Bicester. He is very highly regarded and (in my experience) quite rightly so. If I am not very much mistaken he is a qualified podiatrist to boot - no pun intended. You may however struggle to get an appointment with him before your intended trip as this is now peak boot buying time.
Colin also occasionally works with a chap called Andy Macan who is the sole uk licensee of a device which gauges whether you need shims/wedges to ensure that your skis run flat and that your fore and aft balance is optimised. You sound like someone who might particularly benefit from this.
Good luck
Once you are sorted boot wise, you may want to make your next port of call Jon Coster at the Piste Office in Lambley near Nottingham. Jon is the major supplier and fitter of Dynafit bindings in the uk, and is highly experienced in dealing with requests for shims and whatnot special requirements.