It would be surprising
nonetheless given the debut history of the two franchises. The fact that Odyssey can have the best debut for a Mario game in UK is surprising per se, isn't it? Especially considering Switch is a new hardware. Of course, we have anecdotal evidences that tell us that it might be possible... But possible is not mutual exclusive with expected or surprising. The fact that we can expect this given such recent evidences doesn't make the thing less unexpected in the grand scheme of things, just
less unexpected or surprising.
[Week 46, 2007] ASSASSIN'S CREED (UBISOFT) - 156,000 / NEW (90%)
[Week 47, 2009] ASSASSIN'S CREED II (UBISOFT) - 175,000 / NEW (90%)
[Week 46, 2010] ASSASSIN'S CREED: BROTHERHOOD (UBISOFT) - 207,000 / NEW (90%)
[Week 46, 2011] ASSASSIN'S CREED: REVELATIONS (UBISOFT) - 215,000 / NEW (90%)
[Week 44, 2012] ASSASSIN'S CREED III (UBISOFT) - 332,000 / NEW (90%)
[Week 44, 2013] ASSASSIN'S CREED IV: BLACK FLAG (UBISOFT) - 128,000 / NEW (90%)
[Week 46, 2014] ASSASSIN'S CREED: ROGUE (UBISOFT) -
[Week 46, 2014] ASSASSIN'S CREED: UNITY (UBISOFT) - 153,000 / NEW (95%)
[Week 43, 2015] ASSASSIN'S CREED: SYNDICATE (UBISOFT) ~ 100,000 / NEW (95%)
As for Mario,
Bruno MB had some data. I don't remember where he posted them but they all had Mario debut week below 100k.
That's why I said
historically. You can see from charts already; just compare France and Germany with UK and you can see that UK doesn't have many Nintendo games charting in the top positions unlike France and Germany. Hardware sales are also telling. Just look at Wii U sold in UK versus other European countries such as France and Germany (2015 data but situation hasn't changed much over the years).
Regional differences in terms of consumers preferences and market trends exist. DS and Wii U were huge in UK but they were more outliers than how they were in other countries. Also in such cases, you can see the differences across countries. For example, both DS and PS2 were huge in UK, but the gap between the two was much smaller in UK with respect to France and US.