Responding after skimming - am in danger of being influenced by previous posts!
? Mediation - now a legal requirement, I believe? It's rubbish, in my experience.
In cases where a generally agreeing couple need to fend off ££greedy lawyers, then great! It's the sort of thing good friends should help with, but sometimes life ain't like that and an actual solicitor working for both parties can be a godsend.
But where things are "complicated" mediation can be damaging. I say "complicated" because very few of us, in the moment, recognise that we are being abused. All we know is that nothing seems to make sense ... We go into mediation with a strategy, but the meeting feels like we've gone into Mad Hatter land and the mediator is the Queen Of Hearts 
Mine played lovely husband throughout the sessions, clearly charming the mediator, then staged a full-on tantrum on the pavement outside: he'd agreed to fill out the basic financial forms inside the meeting, but raged that he wasn't divulging his financial info to "any solicitor, bird or bloke"
He didn't, either. I ended up agreeing to some absurdly convoluted deal which left me up to my ears in debt and gave him full control for two years
The mediator's subsequent letters made it clear that she saw me as both the source of the problem and responsible for the charges. That was a result, then 
Sorry, went into a bit of a rant there! To summarise for the visiting reader:
? If you feel confused about your separation, consult an abuse-aware solicitor for advice BEFORE agreeing to mediation.
? Causes - There's still a bizarre amount of conflict around "reason" and "proof" for divorce.
People risked their careers to make it so nobody HAS to stay in a marriage that makes them unhappy. That's it. You don't need to prove anything. For decades already, the law says that if YOUR marriage doesn't make YOU happy, you have the right to end it. Simple? Yes! As it should be.
? You can divorce with mutual agreement that the marriage has broken down. As long as you've been married for a year, either one of you can cite 'irredeemable differences' or 'unreasonable behaviour'. It could be something like opposing political views, badly-matched sexual preferences, different diets, snoring, anything. All the preceding 'reasons' have been successfully cited. The law does not want anybody to feel trapped in a relationship.
[Caveat: I'm 10+ years out of date! If things have changed, they'll have got easier]
? You can divorce with NO STATED CAUSE if you've been married for at least a year and both agree the marriage is now a mistake. This is the scenario that's most likely to benefit from mediation; you can hammer out the details in a non-confrontational environment with legal guidance :)
No-cause divorce, with both parties agreed, takes 2 years to finalise though I think there are circumstances where six months suffice.
? No cause, no respondent: If you leave your spouse, or they leave you, you can obtain a divorce after 5 years of no contact. You do NOT have to show you've made efforts to contact them (a television trope) but it does help if you''ve some documentation to show when the five years started. A good example of documentation would be your divorce application, submitted a few days after you split.
? Annulment is legally available on several grounds, including marriage to obtain a visa. The most historically traditional reason for annulment is failure to consummate (have sex after marriage) but, actually, all governments and all churches recognise other causes for annulment as well. Just not always the same set of accepted causes! Most churches and religions provide for annulment in cases of abuse, disrespect, failure of duty, etc ... however, determination depends on your cleric so you'd be well advised to contact a specialist advisor.
I know all this is really basic, but also know how how impossible it seems when you're locked in the situation. It's all very well saying "if you're being abused" but the truth is, when you're in it, you don't "feel abused". You just feel worn down, out of options, and wrong wrong wrong :(
Post in Mumsnet relationships, phone Womens Aid or Refuge, ring the legal groups linked from their Local pages, consult Rights of Women, and TALK! It will help you get perspective.