Congrats on finding the grave location, thats great work.
I come from the Mays Lane area of Barnet, and still have family there. Visit every 3rd week normally as im 70 or so miles north now.
I would like to find time though to look for Nats grave, though im not familiar with that area. Having moved from London 30 years ago, I still remember the mentality that a couple miles in London is like going on holiday

I do 100 miles a day now just commuting.

There can be undergrowth thats a bit more than sticks an strimmers - hopefully its not like that..... my son and i went for a walk in a local gully a couple months back and when we fought our way through a dead end it was suprising how resilient a few bushes can be!
If there was a way of finding out from the cemetary where the grave is in the block by the grave number, we might at least have some chance of approaching the area, as it would be pretty limiting as to how much you could chop though on a wild goose chase.
Regarding where Dempsey street is, please find new london maps and a link to the 1840 map of london . Dempsey st is pretty much centre of the 1840 map. Ignore the 'pin' on the new map - it looks to me that Dempsey st would have been in the area bounded by smithy st, jubilee st, jamaica st, anf stepney way, today.
http://london1851.com/cross15.htm