The issue arises due to a discrepancy between the DVLA MOT inspection manual, the DVLA number plate rules and the EU document L25/86 Annex XIV "Requirements applying to the registration plate space" General requirements 1.3.1
The EU document should be the overriding authority and 1.3.1 says "Front registration plates are deemed unsuitable for vehicles of categories L1e, L2e, L3e L4e and L5e which shall therefore not be equipped with such spaces! So as an L5e vehicle the M3W doesn't need a front plate and, indeed the Super3 does not have space for one.
The DVLA number plate rules basically say the M3w?Super Three "must not have a front plate"
However the DVLA MOT inspection manual has a 450kg weight limit above which a vehicles should have a plate so this is why some MOT testers will fail you for no plate. The legislation behind this has raised the limit a couple of years ago to 1,000kg so the M£W/S3 are now exempt however this change (as of mid last year) had not been updated in the manual as issued.
So, clear as mud and you can take your choice on how you interpret the need for a plate. I would suggest to call the test station and ask what their position is and if they want a front plate the put one on for the test or go somewhere else. For info, I've had an M3W since 2013 and have never run with a front plate other than putting on on for a few MOTs.
