The procedures are EASA, not CAA. All testing and training is uniform under EASA, there are no national rules anymore. The testfee is the one of the country where you do the test. The FTO has to inform the UK CAA 3 days before the test that it will take place but the booking procedure is as in the country where the test takes place. In fact the examinor is appointed by the FTO now, not by the national authority anymore. UK CAA can issue the license but is not involved in the testing if you do it in another member state.
Contact any of the popular FTO's who do this regularly and they will confirm that rules and regulations throughout the EU are now the uniform according to Parts 1178/2011
If you want to do the test in Belgium then you can if the FE is briefed by the UK CAA. This is an easy procedure that happens online, nothing fancy. He does the test according to EASA rules, you take the paperwork to the UK CAA and they will issue you the license. Done.