Mod 97-10
IBAN check digits are 2 characters wide, which makes them far stronger than a single-digit scheme. Rearranging the string before the division is what spreads the country code and check digits across the calculation instead of leaving them at the front.
Letters in the account body expand to two digits each, so WEST becomes 32143028. That is why a British IBAN with letters in the bank identifier still resolves to a plain integer.
Compute the check digits
Add check digit mode expects the country code followed by the account body with no check digits. GBWEST12345698765432 returns GB82WEST12345698765432.
A valid IBAN can still be unreachable. Length and check digits say nothing about whether the bank identifier is in service, which is what a real payment rail resolves separately.