You absolutely can play from the UK. I would not have mentioned this if I did not know this as fact.
As noted previously, you can only use this from the app due to the licence DraftKings has (remote). People in Ireland are unable to register as the country does not fall into the jurisdiction.
I’ll add an image below showing the iOS app. You’ll see my username to prove that you can play from over here plus you will see that I have money in my account. All done from the UK.