Don’t use agents it’s easy to do yourself. Make sure you answer all questions correctly, Show you can keep her ( money wise ) show she has her own home and a job to go back too. I done it twice. And no problems at all. ( last one took 2 weeks. , em/ year Multi entry for my Thai gf to Uk )
Do it I line, make sure you give as much I do as possible as it’s done thru an agency first. Stare the obvious. Show that gf has a job ( and a home if possible) get a letter from her employer. Show you have plenty of money to keep her , I got 6 month in 3 weeks then a 2 year visa that took 2 weeks , you need to show she will return after visit.