To register a new bike in the UK it requires an MPH speedometer.
DVLA will request an engineers report that the MPH speedometer is present together with the correct headlight.
How do I know - I bought an unregistered bike from Germany - DVLA would not entertain registering it until the above info was supplied, together with HMRC notification showing VAT had been paid, and certificate of conformity.
However it seems that if you forge the document then they may not ask - the KPH speedometer is ok for MOT.
https://www.gov.uk/government/uploads/system/uploads/attachment_data/file/222587/dg_071777.pdf