to fly76737:
if the caa in belgium can issue a jar/fcl license based upon your national cpl, then it shouldn't be a problem working in the uk. i don't think it matters which jaa country you do the atp theory, since you have to go somewhere else to get the exams.
note that a specific jaa country can deny you working there if that country doesn't recognize the jaa country the license was issued from. at least, that is what i've heard.