Letter Before Action hasn’t “delivered”. How to proceed?

We have sent a Letter Before Action, but sent it as Signed For, as this way has been recommended to us several times. It’s been weeks and it doesn’t say “delivered” on tracking. It states something along the lines of, “Your item has been posted at a Post Office. We’ll update when delivered.”

Is it correct to proceed with Small Claims as it’s been a while and we have proof of at least an attempt, or to send another letter with different method? Thank you. (England)