Your history of depression and recurring pnd is not a reason to go on sick leave. What is a reason is if you are not in a fit state to work at the time you should be working. Your history as such isn't relevant to getting you signed off sick, although obviously it gives background and context, what's relevant is your condition at the time.
If you don't think you are fit for work in July, go and see your GP and discuss it, and get him/her to sign you off for an appropriate amount of time. You will not be able to get a sick note for July now.
Having said that are you currently under a doctor's care? If not, I would advise you go and see your GP, discuss how you are feeling, mention that you are due to go back to work in a month and have a chat about your longer term care.
I'm not sure what you mean by a 'ferocious assessment'. Your GP can sign you off work if you are not well and that shouldn't involve any kind of horrendous assessment process.
Having said that, your employer may want you to meet with an Occupational Health adviser, or their own doctor, to discuss your condition, get an idea for how long it might be before you are fit for work again, discuss whether a phased return might be better, lots of things.
Obviously I don't know anything about your condition, or much about depression, but it sounds to me as though it's the thought of going back to work that has triggered this, rather than anything else, is that fair to say? In which case you need to address that - putting off going back to work for a month isn't going to change anything.
Talk to your GP about all this, perhaps see if you can get some counselling as well.
You should also consider whether going back to work is what you want. I don't think it is. You say you can't afford to resign otherwise you would, but your employer isn't going to pay you forever if you are signed off sick - at some point they will want to get an idea of when you are coming back, what they can do to help you back, etc, and if that day never comes they could consider terminating your employment.
If you are signed off sick for a month or whatever come July, what will you do? Do you think you will feel up to work with a bit more time off? I'm not sure. And I think those things are what you need to think about.
Sorry to hear you are feeling so low. Do get signed off when the time comes if you are no better, but try and get some help to work out how to improve things so you can go back to work.