All things considered, it had better be a Hornby A4.
Bachmann's Evening Star (the last steam engine built by Britain's mainline railways) was a limited edition and has sold out.
After that I'm struggling to find locos available as 21st century standard models that are central and iconic enough. Britannias ? Nah , nothing special - not even 8P, just 2 cylinder "standard" light Pacifics, though the Hornby model is good enough. Bullied Pacifics somehow aren't quite iconic enough. A touch idosyncratic , memorable , but not quite the pinnacle
Between the Hornby Duchess and and the A4 , the A4's arguably the better model
Bachmann seem to focus a bit more on the medium sized general service locos - great for layouts but not quite what you're looking for here. You hardly want a WD - the class that everyone forgot or ignored throughout their lives (they even forgot to preserve one though 933 were built....)
The Hornby Arthur is a splendid model , the class gave decades of solid service - but if you start writing a list of iconic British locos, Sir Lamiel won't be first on the list.
Do we have a RTR Star? If so its not an outstanding model
We are short enough of ex LNER locos to need every one we can get, 4472 is almost a pregrouping loco (the first two Gresley Pacifics came out just before Grouping, 4472 was the third, and Thompson deliberately destroyed the first, 1470 Great Northern, as an act of personal spite against Gresley)
There were 80 of them - a pretty big class, outnumbering the entire LMS Pacific fleet , and with 4472 (the only survivor) we have the complete sequence of top link ECML traction from 1870 (Stirling Single No 1) to the present day (including the HSTs and 91s which are still in revenue service) [ Memo to self - I ought to write a small cheque to the "Build a new Gobbler" people]
The fact we're missing a Claude, an LNWR 4-4-0, a Broad Gauge 4-2-2, a Shildon-Newport electric, an NBR Atlantic and an NLR 4-4-0T isn't really the present generation's fault. Nor is the scrapping of LMS 10000 (and the destruction of the Doric Arch)
I'm not quite sure what should be being restored and isn't because of 4472- possibly the APT? The Badger may be better off with the AC Loco people anyway