This muslim chippy owner just told me that halal chippies never use beef dripping, when I asked about what oil they were using.

But as you say, it may not be because beef dripping as such is haram, but rather that the beef dripping that is available and used at traditional English chippies is not.

Honestly, I don't know why. I still look out for the halal sign on chippies, but I've also become better at telling just by the smell.

The beef has to be halal beef. In this context of using beef fat as an additive to other food, the beef fat has to be halal as well.

right, but the the gp said halal establishments always used vegetable oil instead of beef fat.

Perhaps there just isn't enough of a market for halal beef fat to make it worth the while of the carcass processing plants. They might sell it to be mixed with other beef fat.

I can think of another reason too: these chippie vendors might also want to cater to other people who don’t want to eat beef e.g. some Hindus. Some halal Indian restaurants in San Francisco do something similar: they serve halal chicken and lamb, but no beef.

tl;dr Vegetable oil ticks boxes for multiple sets of clientele.

That makes a good deal of sense. I'm a vegetarian, I appreciate it, but was merely surprised given that halal beef and halal beef fat exist. Thanks for these further speculations.