It most certainly was NOT OK.
By your own statement, both trigger the siren sound by default (including to those driving).
By all means do this if a wildfire is heading my way and I need to evacuate, not to tell me I shouldn't be lighting a bloody barbeque! An ordinary text message would have sufficed.
The net result of this is, like most people I know, ALL such alerts are now switched off on my phone. Thus, if Putin does decide to push the button, I'll probably find out about it a little later then you.
You'd need to get the mobile operators involved to build the list of recipients and then to do the actual work of pushing the message out. Normal SMS is point to point, so a message needs to be sent to each subscriber individually which is a huge job at population-level scale.
It makes more sense to use the existing cell broadcast alert infrastructure which has been planned for exactly this type of use case and has been thoroughly tested.
A more reasonable complaint might be that it wasn't geofenced to only the drought-hit areas, but that would only have excluded something like a quarter of the population.
And given that the system does not have that "important but not urgent" category it's also reasonable to complain when the government uses the "urgent" notification system for something that is not urgent.
It caused me to turn off "severe" alerts (leaving "emergency" enabled).
But, to be honest, it doesn't seem to be too far out of line with the past alerts listed on https://www.gov.uk/alerts/past-alerts
(I also only have the extreme alerts enabled - I've been disabling severe ones since the system was introduced. If there's a storm I'm likely to be staying inside anyway!)
Both levels can be deactivated on Android (no idea about iOS but I presume it's the same), there's no unblockable presidential alert level.