I haven't done the GDL but can answer some of your questions anyway.
There is not a massive difference in course providers. If you want to save money those outside London are cheaper. On the subject of saving money if you want to work as a solicitor after qualifying then some firms will pay for you to do the GDL and LPC, in which case they will tell you which provider to go to. You can apply for Training Contracts next year (someone will correct me if I'm wrong).
re. A-level / degree classification. Technically you need a 2:2 to do the GDL. I'm sure if you had a third they would let you do it as long as you had a cheque for the course fees in your hand. The GDL providers have no particular reason to weed people out on their academics. The more important issue is whether your degree classification / a levels will be enough for employers. Some law firms require AAB+ and 2:1s (probably some actually require a first but don't say that they do).
History is a fine subject to go into law from. Look at Jonathan Sumption for instance. But really, any academic subject is fine and history undoubtedly is academic.