Campervan 240v first fix wiring

Here i explain about how I first fix wire my 240v system in my van but then i discovered the correct method too late and had to re-wire the system again using the correct wire.
I was told 1.5mm cable is more than enough from an electrician friend of mine who worked for swift caravans many years ago but i did fit 2.5mm flexi core in the van which I’m glad of now because i will install a seperate 16A socket and feed at home running from the house consumer unit which will power a 2.5mm hook up lead. This will give me at home a huge power consumption of 3.7kw when hooked up on my drive. This means if i have guests over i can house them in the camper, provide them with electrically heated hot water whilst also keeping them warm with an electric heater. Plus they can use the microwave, tv etc pretty much all at the same time without tripping the system. Also im not sure if its now the uk regs for 2.5 cable but best do your own due diligence or go for playing it safe and run a 2.5mm ring around your van!