this is half correct. For licence issue, the state of medical issue must match the state of licence issue. So if you did your initial medical class 1 in Spain, all you do is to do a renewal when the time comes up for licence issue in the UK. - As this is a long time to go with GS and CPL training to fit in, you would be up for renewal anyway.
After licence issue, you will be able to do your medical wherever you want in JAA land.
Most CAAs handle it this way, some are more lax, but not the UK CAA with a Spanish medical.