Aside from HMS Canada being a one off ship originally built for Chile, a couple of other points in favour of Iron Duke in no particular order:
1. Iron Duke is named after 'The Iron Duke' AKA Arthur Wellesley, 1st Duke of Wellington, who defeated Napoleon at Waterloo.
2. HMS Iron Duke duelled with SMS König at Jutland, having her in-game would allow them to repeat their historical battle.
I wonder if it would be technically feasible to make HMS Agincourt, HMS Canada, and HMS Erin multi-nation premiums (for the appropriate nations), possibly even changing their names depending on which captain you use?
Also while I favour HMS Vanguard for the tier 8 slot, I haven't seen anyone bring up the 1938/1942 Lion designs, while the 1944 designs are easily tier 9 the 1938/1942 actually fit quite well at tier 8. The UK 16" Mk II/III penetration is similar to the US 16" Mk 6. The main battery DPM is slightly better than Bismarck/Tirpitz but worse than other tier 8 BBs. The HP would fall in at tier 8 as well and the armor scheme is almost the same as HMS Vanguard and the KGV class.