To check that all of some input matches a pattern (not just part of it), use re.fullmatch. It's the right tool for validating input:

import re

def valid_postcode(text):
    return re.fullmatch(r"[A-Z]{1,2}\d[A-Z\d]? ?\d[A-Z]{2}", text.upper()) is not None

for p in ["CT1 1QU", "sw1a1aa", "ME14 2LP", "12345", "CT1 1QU extra"]:
    print(p, valid_postcode(p))

re.search would accept "CT1 1QU extra", because a valid postcode appears inside it. fullmatch requires the entire string to match.