Deadline for defendant responses is calculated from the date of service which is not the first date of claim.
According MCOL guide for Claimants, date of service is the 5th calendar day after small claim issued. If additional particulars of claim would be sent separately it affect on calculation service date.
At the same time second page of the form N215 contain information that deemed day of service claim is the second buseness day based on CPR 6.6(14):
Deemed service
6.14 A claim form served within the United Kingdom in accordance with this Part is deemed to be served on the second business day after completion of the relevant step under rule 7.5(1).
What method to serve claim to the Defendant?
Companies, partnership or individual may served by any method that specified in Court Procedure rules 6. Claimant may served by e-mail, 1st class post with delivery to the next business day, personally or by fax. All method is explained in CPR 6.3 As well court may authorize any other method.
Court rules don’t highlight wether parties must or should use recorded delivery service in case of choosing Post method. But there is general recommendation to use recorded delivery with tracking code that allows to track served documents and have the proof of its receiving.