Get your documents together, birth certificate, NI number, passport, marriage cert, kids documents (of you have kids). Keep them somewhere safe preferably at a friend or relatives house. Do the same with anything that is important to you such as photos etc.
If you can, stock pile and store with a friend/relative. Add bits to your shopping every week but not so that he’d notice the cost going up. Get as much stuff as you can e.g towels, bed linen, cleaning stuff, dried and tinned food etc.
Talk to womens aid or local dv charity to get a safe exit plan.
Always keep an overnight bag handy incase he works out what you’re doing and you have to get out quickly.
Start to sort out where you are going and how you are going to get there.
Do not tell him that you are going to leave.