add to theme functions.php file to add uk as an available format on the phone type field and validation of uk style numbers, pattern from: http://www.coffeecup.com/help/articles/regular-expression-examples/#phonenumber-uk

validate_uk_phone.php

<?php
//checks to see if a string is a valid UK phone number
add_filter( 'gform_field_validation', 'validate_phone', 10, 4 );
function validate_phone( $result, $value, $form, $field ) {
	$pattern = "/^(((\+44\s?\d{4}|\(?0\d{4}\)?)\s?\d{3}\s?\d{3})|((\+44\s?\d{3}|\(?0\d{3}\)?)\s?\d{3}\s?\d{4})|((\+44\s?\d{2}|\(?0\d{2}\)?)\s?\d{4}\s?\d{4}))(\s?\#(\d{4}|\d{3}))?$/";
	if ( $field->type == 'phone' && $this->phoneFormat != 'standard' && ! preg_match( $pattern, $value ) ) {
		$result['is_valid'] = false;
		$result['message']  = 'Please enter a valid phone number';
	}

	return $result;
}

Hi Richard,

Thanks so much for this snippet. It is excellent except in one minor detail - the field is invalid even if left empty. I would like the field to be valid if empty (ie not a required field). Is that possible? Thanks.

Just following up on this, if you add ^\s*$| to the pattern after the "/ then the field can be empty, or contain any amount of white space.

Hi, just tested this on my clients install, however, whenever I choose UK and click save, it jumps to this option "(###)###-####".