Hmmm, like strictly speaking you will have to pass an SVA test as its a major chassis modification

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sure you can Tax, MOT and Insure it, because insurance companies are happy to take your money, and you can declare the modifications. However if you have an accident they could use the lack of SVA as a getout clause

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fortunately the commercial SVA test is a breeze compared to a passenger vehicle one. you will be able to retain the original registration number as its still the same vehicle stretched a bit, so no Qs.
however if the modification was carried out before the introduction of the SVA you dont need one

but i believe its up to you to prove it when it was done, with documentation.
I guess most people just wing it and get away with it. i would say just about every landrover rangerover hybrid in the country hasnt been SVA tested but should be because the chassis will be shortened and the back crossmember changed to a landrover one .
check out the NSRA website and DVLA before you take my word for it. you can make up your own mind!