Depends really.if you do what your planning and register it properly it will get a q plate from the peeps at the DVLA.
you can just not declare any changes and ride around but if you have a crash and its checked your insurance is void and if your stopped by the police they will not be able to tally your cars ID to the licence plate so likely will get took off ya.
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On another forum a member had his galv chassis Defender stolen and quickly recovered by the police, I know unbelievable!
Anyway, you are right the police would not give it back until he provided proof of the chassis change. Once done they handed it back no problem.