my advice would be to forget deadlifts for now, at least until you have the core strength to keep from blowing your back out.
learn to hinge properly, pavel tsatsouline has some really good drills involving bodyweight sumo-box-squats to jumps. practice hinging with a movement less taxing. look into kettlebell swings, good mornings, romanian deadlifts.
most of all ask someone to watch you and stop whenever form deteriorates at all, until you are able to do the movement safely and you develop the prerequisite strength.
you'll see a lot of elite level lifters with slight rounding of the back, ranging to some pretty extreme rounding in some cases with maximal weights. the only way to do this safely is to develop the necessary mid-back, core and stabilising strength to keep the rounded spine from folding in two.
sorry to keep harping on but until you have the necessary strength to do this safely you need to put your back in the safest possible position, which is with a near as damnit neutral spine.
as an aside, i would guess your hamstring flexibility is lacking, which is part of what is preventing you getting down into that position with a flat back.
p.s. youre english is perfect