You’ll have to show she can stay with you ie landlord letter of approval. Send her an invite letter. Make it clear you will cover her costs and medical insurance.
Any correspondence or statements you’ve sent money to establish how long you’ve known her. They just don’t want her to be a burden on the state and go back when she says. List reasons she has to go back, job family etc
don’t use agents. I have had g/f visitor visas, spouse visa children visa and never used an agent and never refused. Do the homework and provide the documents and no problem.
They make money from visas they just want to make sure there is no burden on the state.
If you can pay for her then you will be her sponsor therefore she won’t need financials. If you are staying with family you might need an invitation letter to support where you will be residing whilst there.
It’s a good idea to take out an insurance policy in case she falls ill for the time you’ll be in the Uk.
You just need to satisfy them she won’t be a burden on the state and leave when you say you’ll leave.
You just have to provide the documentation asked for. They need to know she has somewhere to stay and enough money for the trip or you will and she’ll return when you say. I got mine medical insurance for the uk as well for the trip. They make money from visas so as long as you satisfy the requirements there shouldn’t be a problem.