If you’re not fitting beds
If you’re not fitting beds then you’ll have to leave it as a “van”. Beds are a requiremnt for re registering as a camper. You should be able to reclassify it as a “van with windows” but this will only affect the MoT. My insurance company were happy to insure my minibus as a van when i explained that there were only 8 seats in it. For resale value, it might be better to leave it registered as a minibus, because someone might want to convert it back after you sell it.
PS, making a “dayvan” is a great idea, your kids will love it and when you sell it on, you’ll have plenty of interest from fishermen and surfers, who basically only need a mobile beach hut 😉