I read something the other day:
This reminded me of a photo that was sent to me some time ago.
One Muslim brother sent me and many others a voucher for Burger King. I replied to the brother and asked him why he sent it to me and asked who even goes there.
He then asked me to read the small print and said that he didn't eat there himself, but knew of people who eat their Fish Burgers!
The small print said,
"*Product is cooked in the same oil as fish and chicken products" Source
I understand this topic can go a long way and include many other fast food outlets, therefore, I would advise everyone to do their own investigations should they wish to eat fish burgers from other places.
Please note, each outlet has its own rules and regulations, some fry in the same oil while others don't fry in the same oil. Larger outlets have bigger facilities to abide by those rules, smaller facilities tend to take short cuts and break rules. So even if the ingredients are all Halal in the Fish burger, just make sure the end product is also Halal.
This is to confirm that Burger King's Fish is Haram.
As mentioned above, I have yet to meet someone who has eaten from Burger King in the UK :)