If you send a registered letter, the tenant may refuse to sign for it. However, if you send it via "Swift" (I think that is one of the names for it), the Post Office guarantee delivery without having a signature on receipt.
There is no law on how long the "warning" letter should be sent before the 14 day notice is sent. If there is a fixed term agreement in place, there is no need to send this warning letter; it only applies to Part 4 tenancies.
However, it is no harm to send one and advise that if full payment is not received within, for example, 3 days proceedings will immediately commence for both eviction and the recovery of the rent arrears. Then send the 14 day notice after the 3 days - there is no need to wait for a week. Some landlords follow with the 14 day notice just a day or two after the warning letter.
SW rent reviews for the tenant are not the concern of the landlord. If rent is not received in full and on time, the tenant is in breach of their rental obligations. IMHO, too many landlords do not initiate rent arrears proceedings soon enough especially as the tenant could have 6 weeks free accommodation (if they vacate by the end of the notice of termination).