Hmmmm are Spain and Italy similar topography wise? From what I've read it seems that had the British concentrated on Italy rather than Hanover they could have negated the French advantage in artillery and cavalry.
Italy is better horse country in parts, although the hills in parts make it very infantry orientated. The problem is likely one of shipping, Italy is a lot further away than Hanover.
Also, at least in Germany there's a chance of recalling the troops for home defence. Those in Spain, Italy etc. are gone for that purpose.
If the British choose, and there is sufficient time to allow the movement, the British could (by sending the entire disposable force into Hanover) put 50,000 bayonets, 10,000 sabres and several hundred guns onto the continent in 1805-6.
This is a very significant force indeed. It is likely that it's commanders (drawn from the home station) may include:
Army Command and Staff
General Sir David Dundas (GOC South)
General Francis Earl of Moira (GOC North Britain, possible 2i/c)
Division (or Corps) Commanders:
Lieutenant General John Earl of Chatham (GOC Eastern District)
Lieutenant General Francis Dundas (Commanding Army Corps, Kent)
Lieutenant General George Don (Commanding Army Corps, Sussex)
Lieutenant General HRH Ernest Duke of Cumberland (GOC Southwest) - note this is the future King of Hanover
Lieutenant General Sir Hew Dalrymple (GOC North)
Lieutenant General Harry Burrard (Commanding a Division in London)
Lieutenant General HRH Adolphus Duke of Cambridge (GOC Home)
Lieutenant General Banastre Tarleton (GOC Severn)
Lieutenant General Richard Vyse (GOC York)
Lieutenant General Francis William, Duke of Gloucester (GOC NW)
Lieutenant General John Simcoe (GOC West)
Lieutenant General Francis Gwynn (GOC South-Inland)
Major General Arthur Wellesey served in the real 1805 Hanover expedition as brigade commander, and would do so ITTL