
The risk is no less, just because they are on vacation. So fewer RPOs would be a serious risk. Apart from the fact that fewer ROPs means more work for the rest. And more work means more tired and less alert RPOs who may eventually be prone to cut too many corners. - Until one day...
Since it's the police and security organizations who asses the threat-level for the individual members of the BRF, it's also the police who determine what level of protection is needed. It would IMO be dangerous to let costs outweigh the threat-level. Also for the protections officers as well as bystanders.
It's logic: Less protection means a higher probability for someone to try to carry out an attack or a kidnapping attempt. Simply because there is a higher probability for a success.
Also, if costs is a decisive factor in providing protection that will apply to everyone. If you cut down on RPO officers you create a good argument for cutting down on other protection jobs the police has.
The natural reaction to that is to take matters into your own hands. I.e. for those who can afford it to either leave Britain or hire private contractors to do the job. Contractors who may have an inferior training or just as bad; be more heavy-handed or even trigger happy.
Another alternative is for people who feel threatened to arm themselves, regardless of whether that is against the law or not.
And why should people who were born into a high-profile, high-risk position like the members of the BRF wish to continue if they feel they don't get adequate protection?
Being royal wasn't something they chose themselves.
Let's toy with the idea. Harry and Meghan get less protection. That saves money and works fine for two or three years. One day, on a vacation in Scotland (because other countries will make sure they are protected!) they are attacked. Whether they are killed, wounded or unhurt is besides the point.
In this scenario however a film of Meghan held hostage released by The Revolutionary Action Committee of Left-handed Toyota Dealers is shown non-stop on every channel...
The subsequent inquiry will show that the amount saved on RPOs is negligible. Certainly in comparison to the huge embarrassment Britain is facing whenever that film is shown.
And those who were most vocal in suggesting the number of RPOs being cut will be lucky if they are hung from the Tower by their necks!
So no one in charge will cut down on royal security, because no one in charge will be remembered for being responsible for a member of the BRF being hurt, killed or kidnapped.