When do kids switch from pencil to pen??

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DS is in Y5, in a British school, but we are not in the UK.
His writing is terrible (and spelling, and yes, SENCO is involved).

All school work is done in pencil.
He picked up a biro yesterday to practice his spellings, and WOW, what a difference!

So, at what stage do kids typically switch from pencil to pen in the UK?

We don't give them at all. There's no expectation given by the government (as some PP have suggested) that children need to be able to write using a pen.

France here. Writing in fountain pen from age 6. Lots and lots of work done on writing in cursive.

Graded every other week for handwriting.