Short answer: No. Gibraltar pound coins are not legal tender in the United Kingdom. Here's why—and what actually works.
In UK law, "legal tender" has a narrow technical meaning: it's about settling debts, not everyday purchases. Crucially, only Bank of England coins (and Royal Mint issues) are legal tender across the UK. Scottish and Northern Irish banknotes aren't legal tender either—but they're widely accepted by convention. Gibraltar coins fall outside even that convention.
Gibraltar-issued banknotes (printed by the Government of Gibraltar) are technically not UK legal tender either, but they're more widely recognized due to their parity with sterling and similar design. Still, acceptance is at the merchant's discretion.
Gibraltar pounds are pegged 1:1 with British pounds sterling. The coins look similar (same denominations, similar size/weight, Queen's effigy). But they're issued by a separate monetary authority under Gibraltar's Currency Notes Act. Think of them like Isle of Man or Channel Island coins: same value, different jurisdiction.
Don't rely on spending Gibraltar coins in London, Manchester, or Edinburgh. Exchange them before you cross the border—or keep them as distinctive souvenirs of a unique territory.