So here's what we know. Overall sales are down 25%. Game was on sale during its first week this year for 3 days less then last year. Digital percentage would also have increased. 

PS4 retail sales ~500k. How many did it sell at retail on ps4 last year? 

As far as I know the PS4 sold more (from 53% to 60%) this year than last year and don't forget these are 4 days earlier than last year.

As per Bruno on Gaf, Fifa 18 did 499k on ps4 (at retail) as opposed to 587k for Fifa 17. 

X1 did 307k for Fifa 18 compared to 443k for Fifa 17. 

Plus the 360 and ps3 versions went down again. Without digital numbers, though, it's hard to know what the drop actually was among each system. Could be more, could be less. 

This is the first year that PS4 has the promotion? so sales should be up for PS4 down for xbox one.

(http://www.vgchartz.com/game/117272/fifa-17/UK/):

UK First Ten Weeks (Units)

The top 3 best selling games currently on the EU store are all FIFA 18 with the ICON and Ronaldo editions charting above the regular edition. So yeah, more than £60 I guess.

OP is specifically referencing UK sales.  Where are you guys getting that there were 3-4 days more sales data in last years release?  Fifa 17 only released early (Sept 27, 2016) in NA.  It release WW (UK included) on Sept 29, 2016.  September 29, 2016 was on a Thursday last year, 1 day earlier than the September 29th, 2017 release of Fifa 18.

https://www.easports.com/fifa/news/2016/fifa-17-release-date

Last year's was the first one to use the frostbite engine and have a story mode.

This years is just more of the same, not much in terms of progress, so it's not surprising that it would see less. The story mode didn't turn out to be that good either and people know that now.

Digital starting to eat retail share.

Digital > Physical.

I bought the game digitally and so did all my friends that play FIFA. Last year of few of them bought it physical.