The actual number was £347,250, which is £17 over the nearest prime number. In USD, the actual number was $437,955 (headline rounds down for some reason), $2 over the nearest prime. They were probably limited to 250$ differences, though, and 347 is prime, so that's something.
Primes are pretty dense. So any random amount will be close to a prime number.
The enigma's encryption didn't have anything to do with primes though :) It was just a super complex automated substitution.