Like I said, this was drawing from the Hornby Products Database, so it's likely out of date/not fully complete, but according to it, the following A4's have not been modelled for mass retail by Hornby in OO gauge:
4499/60002 Pochard/Sir Murrough Wilson
4462/60004 Great Snipe/William Whitelaw
4496/60008 Golden Shuttle/Dwight D. Eisenhower
2510/60015 Quicksilver
2511/60016 Silver King
4900/60032 Gannet
My information, again, is likely to be inaccurate, and doesn't take into account if the engines were renamed at any point in their careers; for instance, I've seen models made up as 60034 Lord Faringdon, but not of her original form as 4903 Peregrine.