Validation rule

As @hmitch2 discovered, UK postcodes are alphanumeric, not just digits. The usual structure is an outward part (2–4 characters) and an inward part (always 3 characters: one digit then two letters). That means, excluding the space, postcodes are 5–7 characters long; including the space they are 6–8. For a database you can either store a normalized form (no space, uppercase — set CHAR(7) or VARCHAR(7)) or store the displayed form (with space — allow 8 characters). As @cpopham suggested, set the field size in your table; as @DanceInstructor suggested, use a pattern check for format validation rather than relying only on length.

A practical, commonly used pattern that catches the usual formats (but not every historical exception) is shown below. Use it to validate input and normalize before storing. For definitive validation against actual assigned postcodes, validate against a postcode dataset or API (recommended).

^[A-Z]{1,2}[0-9][0-9A-Z]?\s?[0-9][A-Z]{2}$

To normalize before storing (example SQL), remove spaces and uppercase:

UPDATE addresses
SET postcode = UPPER(REPLACE(postcode, ' ', ''));

For production use, consider a complete authoritative pattern or a lookup service (for example postcodes.io) and consult the full format rules (Postcodes in the United Kingdom). If using MS Access for a GCSE project, set the Field Size to 7 for normalized storage (8 if keeping the space) and validate more strictly in form code using VBA RegExp.