Does my girl fiend from Ukraine need a visa to visit the UK??

Yes, she will need a visa and it is difficult to get. The ultimate decision on whether she gets a visa or not is based on a mid-level government bureaucrat who can cite any of a thousand reasons why he/she won't grant a visa including the bad haircut they got at lunch. Both the US and the UK embies in Kyiv rely on the same decision-making process--you must PROVE beyond all shadow of a doubt that you will leave the US or UK at the end of your visit. This is virtually impossible for most people to do. You can try, of course. Maybe that bureaucrat had a very nice haircut that day.