Also a valid point I forgot to mention.
When spending on the Starling Euro account, you do the conversation at the start of your holiday - Say, £1000 = €1100
You then spend in € and don’t worry about when the transaction finally settles in 2-3 days time, or any sudden fluctuations of currency (Brexit related or not).
I should think this would be relatively minor for a family holiday in Europe - But for those who like to budget, and want to know exactly what they are spending, it’s a good way to go.
Obviously the currency fluctuations can also work in your favour, so it’s not always best to convert beforehand - But if it was me, I’d probably do so for sheer peace of mind (especially as it won’t vary much over the course of a week).