It gives you a UK CPL with ATPL theory credits once you attain the required number of hours. 1500 of them I think.
You can just do the CPL if you want, but if down the line you decide that you want an ATPL there is no credit for having already done the CPL exams. You will need to sit all 14 ATPL exams (expensive!), although I think there is credit for amount of groundschool you will need to complete.