I think that your friend strictly observing Halal but nothing else is very much a Pakistani thing. I have noticed that Pakistanis are very strict about halal, even if they are not strict about anything else. I have noticed that Arabs, Iranians and Africans are a lot less fussy that Pakistanis! In my first year of uni, there was this Nigerian girl I lived with who would buy her meat from Morrisons and say it is ok if you say "Bismillah" before you eat it, and me being a Pakistani, found that attitude very peculiar.
My cousin used to have a chip shop and he would get very annoyed with Pakistanis coming in saying "Are you sure your meat is halal? How do you know? Where do you get it from"!
Anyway, what I am trying to say is, Pakistanis seem to think that the absolute worst thing you can do in Islam is eat haram food!
Well IMO the best burger available in the UK is Frankie and Benny's
Black Pepper Mayo Burger. It's relatively expensive so I don't go that often.
But a cheese whopper with bacon will do
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The best burgers I have ever had are from the Gourmet Burger Kitchen - The Greek Lamb burger is absolutely gorgeous! Another restaurant is brilliant for burgers, and I think it is worth travelling to Northampton just to go to this restaurant called "Buddy's" - I had a lasagne burger from there which was so nice!