The Letter Before Action is a formal notice which must be served properly.
If the debt is owed under a contract check that contract for any special rules about serving notices and make sure they are followed.
The safest way to serve notice is to deliver in person or failing this, to send by courier, post or email.
In person: hand the Letter to the recipient personally.
Post: if you are serving the Letter by post it is recommended that you use recorded delivery and that you keep a copy of the proof of posting and delivery.
Email: check that the contract allows you to send notices by email and whether it requires you to request a ‘read receipt’ as proof of service.