Form validation which simply checks for a blank field is barely worthy of the name, and virtually useless, as even a single space, an X or a ? will return false, that is pass the validation. Numeric values, such as zip codes and phone numbers, should be validated as such. Ditto email addresses. This topic has been covered many times before in this forum.
To be candid, you would be best advised to ditch that elderly generic (and inefficient) validation script and write a proper one.
Not all UK telephone numbers are 11 digits. e.g. (01204) xxxxx Bolton
For phone validation see e.g. http://www.codingforums.com/showthread.php?t=156981
Quizmaster: The small primates called lemurs are native to which large island off the coast of Africa?