There were some pre-Roman coins which used the Latin alphabet for names of kings such as Tasciovanus and Cunobelinos, but no native script that I've come across.

Commenters:

• Please stop saying "runes". It's not the correct answer.

• Please stop posting the Wikipedia page about runes, which itself also says it's not the correct answer to OP's question.

• Please stop commenting about there are a lot of deleted comments. There are a lot of deleted comments because people keep giving short answers saying it's runes.

Next person to say the answer is runes had better have some pretty detailed and conclusive evidence that they're not just throwing out half-remembered things they once heard after reading the Hobbit. Otherwise you're going to get a cranky reply from me.

/u/ctesibius has already addressed the question of runes.