Is my passport valid should I opt for reissue.
Many passports have the passport number perforated on all interior pages of the passport as a security feature: it allows detection of pages exchanged from another passport (page numbering allows detection of added/removed pages).
Sometimes there are two different numbers in a passport: a booklet number (which is the serial number of the specific booklet and is decided before the passport is actually issued) and a passport number which is decided when the passport is actually issued (in some countries that number contains the year of issue and/or codes identifying where it was issued). In that case it would be the booklet number which is perforated, at the time the booklet itself is printed and assembled (but still blank).
See for instance UK passports introduced in 2006:
9 digits, printed on page 1. These are entered on the bio data page in the same style as the personal details. The serial number is perforated through pages 1-30 (1-46 in business book).
(...) The holes of the laser perforated number are larger at the front of the book and reduce in size to the back.
The rules are different for later versions of the UK passports (some have the rear page perforated as well, some have the number engraved on the title page, some have a check symbol, etc.).