Lots of jobs and graduate schemes accept any degree and History is a solid, well respected one with lots of transferable skills.
Lots of Historians go down the Law route, they go down lots of routes.
Your prospects are only poor with a History degree if you let them be, think about what you want to do and then make the effort to get work experience in that area, make yourself employable.
People with an LLB have it just as difficult when finding jobs as anyone else these days, your prospects won't suddenly become secure if you change degrees.
Is it definitely Law that you want to go into? Or are you just looking to do that because you think you're more likely to get a job? Because that isn't guaranteed. You should also think why it was that you chose to do a History degree in the first place, I'd presume that it must have been because you enjoyed it and wanted to study it for several years, aren't you sorry to be giving that up at all?