50 Pence - Elizabeth II Tommy Flowers

Tommy Flowers, the GPO engineer who built Colossus at Bletchley Park — the world's first programmable electronic computer — spent £1,000 of his own money completing it after the government refused full funding. He was never reimbursed. Tristan da Cunha, a British Overseas Territory with a population under 250, issues coins primarily for the collector market rather than circulation, making this effectively a commemorative medallion in coin form.