It's £18,600 if no dependents.
Yes, you need either a job contract or an offer letter starting in the next 90 days to move together without a separation. As long as you've made over £18,600 in the prior 12 months (rolling period), there can be a gap so it's perfectly okay to have been unemployed for a bit.
When returning, most people get permission to live with a friend or family member. Otherwise, you'll need a longer term housing solution. Corporate style housing for a couple of months works as well. They just need to know you aren't going to be homeless.