Black Pudding - large, flat mushrooms chopped and cooked for a very long time until they are jet black, then combined with oats, lots of pepper, spices and some fat and maybe some finely chopped pinenuts would be the nearest thing.
Trying a veg*n haggis isn't quite the same, as a haggis tastes different and has a looser texture, but it would be the nearest, perhaps with added mushrooms?
It's an intensely deep, rich and savoury flavour with a crispy outside and softer texture inside.
Lobster, I have no idea. There is nothing that can replicate prawns, which are sweet and light and savoury and moist and dense all at once, so I'd imagine it's equally impossible to copy Lobster.
Mock crab is a reasonable imitation of the real thing, though. Sieved boiled eggs, grated cheese, salad cream/mayonnaise, vinegar and a load of pepper. Perhaps try that but add a little ketchup to the sauce to get something more like shellfish? It's pretty much what crisp manufacturers have for things like Skips.