I don't knwo the exact rules that define a valid UK phone number and as you have not provided any rules to validate a Phone no, I just picked rules listed in the url provided by Colin Pickard in his answer.
The following rules are checked for validating UK phone number -
1. Telephone number provided is not null
2. Telephone number does not contain the required 10 or 11 digits.
3. A valid full UK telephone numbers must start with a 0
If there are any rules that I missed out, you can add the check for those conditions too in this function.
ALTER FUNCTION [dbo].[ValidatePhoneNo]
(
@PhoneNo varchar(20)
)
RETURNS varchar(10)
AS
BEGIN
DECLARE @Result varchar(10)
SET @RESULT = 'invalid'
IF len(@PhoneNo) > 9 AND len(@PhoneNo) < 12 AND @PhoneNo IS NOT NULL AND (substring(@PhoneNo,1,1) = 0)
BEGIN
SET @Result = 'valid'
END
RETURN @RESULT
END