This isn't how it works. Just so we aren't giving OP misinformation.
If a crime has been committed (which it has), it will be dealt with the same way no matter which method of contact OP uses. 999 doesn't automatically mean arrest. It will just get her someone to speak to sooner.
101 is non emergency, so meant for crimes not in progress. This is in progress tbh, as he's still in the house. That's why 101 is a bit slower.
Online contact forms are the same as 101, meant for non-emergency so a bit slower.
However logs come into the police, whether 999, 101 or online, they will be graded. People can call 999 for non urgent stuff just like others will be too scared to call 999 and use 101 instead. They are all looked at and assess the same and graded in priority for response.
But OP whatever contact you can make with the police will be helpful, do whatever you are comfortable with.