There is no reason why a school should prevent you from doing one module at a time. If you choose one that runs a 3 module course each module will typically be about 2 months long. Provided you can get time off work for 2 months at a time you can spread the 3 modules over a year or more. You must of course take care to avoid breaching the 18 months limit for exam attempts. But I suspect that most employers would not agree to such long periods away from work.
If however you mean doing one of the 14 subjects at a time then most schools would have problems providing it.