What's the national (food) dish in the UK?

It's very hard to define a national dish for all of the UK, as there are certain dishes that are either country or region specific (such as haggis being favoured in Scotland, or indian dishes in areas such as Leicester where there's a large asian population) so what's best is a country breakdown.

England: Fish & Chips (usually cod)
Scotland: Haggis
Wales: Laverbread (a strange seaweed thing)

A possible UK dish could be a Sunday Roast, pictured below.

Yorkshire puddings, mashed potato, roast beef, gravy and plenty of vegetables are the main things in this dish. Very tasty.

as joe mentioned above there are a few place specific dishes but it's funny you should ask this as i was just watching a cooking competition show and Jilly gooldon said the most home cooked dish in Britain is actually shepard's pie, not sure if you have that in America but it's pretty must just lamb mince often with carrots and onions and then mash pototoe on top. I did try and put up a photo the only trouble is i couldn't find any that looked appetizing.

anyway while chicken-tika masala is very popular in britain and as is fish and chips both originate from asia (although i think it was healthy grilled fish and baked potato slices than the unhealthy fried stuff we now have lol)

so yes i would say Shepard's pie or possibly as Joe pointed out a good Sunday roast, other typically english dishes would be things like toad in the hole (sausages in Yorkshire pudding) or bangers and mash (sausages and mash potato)
I think we are pretty much a nation of people who love any thing with potatoes in lol

chiken tikka masala as well meat pies and fish an chips

you are correct dont know why but yo are correct bro