Can the Greek PPL be validated by the UK CAA and have a UK IMC placed on it via the validation or is the rule simply ' to have a UK IMC, the PPL must be studied for and passed in the UK' ?
No. First of all you need to do the Greek JAR-FCL compliant theory exams (presumably these are in Greek!) and the Greek JAR-FCL compliant skills test (probably with a Greek-speaking examiner). This will give you a JAR-FCL compliant PPL which is administered by the Greek authorities.
Assuming you live in the UK more than 180 days a year, you can then trade in this Greek-administered JAR-FCL compliant PPL for a UK administered, JAR-FCL compliant PPL. This will cost you nothing more than an admin fee. Follow Beagles link.
There is one advantage to this and that's that on that UK administered PPL you can add an IMC rating, which you cannot add to a Greek PPL (because the Greek have no idea what that is).
But the disadvantage is that the ANO limits your PPL (sans IMC) a bit more than the original JAR-FCL regulations. Whereas JAR-FCL is quite happy to let you fly VFR-on-top, the ANO is not. You can only fly VFR-on-top once that specific restriction on your PPL is lifted, and that only happens if you have an IMC or IR.
The no-VFR-on-top is a restriction placed by the ANO on your PPL so that restriction is also in force in countries that normally allow VFR-on-top for their PPLs and their airspace. For instance France.
There's also an alternative solution. You can do all your theory exams plus the R/T practical in the UK. You then go to Greece to learn how to fly. Make sure all your flight hours are properly recorded in your logbook, including exercise numbers, signatures and so forth. Then do the skills test in the UK. This saves you from having to pay license issue fees twice.