Five Guys sells burgers, all-beef hot dogs and fries cooked in peanut oil. In the UK it answers the Halal question itself, store by store, on a page called Halal Information.[1] The answer comes in three groups, and which one your store is in decides it.
The three groups of UK stores
Fully Halal-certified. Three stores, Birmingham Fort, London Park Royal and Manchester Arndale, are certified by the body Five Guys names as HCO. They serve Halal-certified British beef, sell no pork bacon and serve no alcohol.[1] This is the clearest answer Five Guys gives: Halal meat in a store with no pork at all.
Halal beef, bacon on a separate grill. Twenty-nine stores serve Halal beef but also sell pork bacon. Five Guys says the bacon is cooked on a separate grill with separate utensils, and that these stores are not fully certified by HCO.[1] They include London Bond Street, Brixton, Croydon Valley, Kensington High Street, Queensway, Wimbledon, Wood Green and both Westfields, and stores in Birmingham Five Ways, Cardiff, Edinburgh, Glasgow, Leeds, Liverpool, Manchester Fort, Manchester Parrs Wood, Nottingham, Sheffield and elsewhere.
No Halal beef. "The rest of our stores in England, Scotland and Wales are not Halal certified and do not serve Halal beef."[1] At those stores the burgers and hot dogs are not Halal.
At its Halal stores Five Guys serves the hamburger, cheeseburger, all-beef hot dog and cheese dog, the grilled cheese and veggie sandwiches, fries, milkshakes and soft drinks.[2] It says the hot dogs at those stores are made with Halal-certified beef, and that the fries are cooked in peanut oil.[1][2]
Alcohol and one inconsistency
The fully certified stores serve no alcohol. Many Halal-beef stores serve Hawkstone beer. Five Guys' own FAQ lists London Bond Street and London Queensway both among the stores with no alcohol and among those serving beer,[1] so if that matters to you, ask the store.
Is a Halal-beef store enough?
That is a question of standard, not of fact. The meat is Halal; the kitchen also handles pork, kept apart by grill and utensils. Many Muslims accept Halal meat prepared separately in a mixed kitchen; others want a fully certified store with no pork on site. If you want the second, the three HCO stores are the only Five Guys in the UK that meet it. What a certificate covers is explained in How to check a Halal certificate.
Outside the UK
Five Guys also runs sites for other countries, including Saudi Arabia, the UAE, Kuwait, Qatar, Bahrain and Malaysia. This page covers only Five Guys UK's statement; check the Five Guys site for the country you are in.