The warranty gives more than CRA 2015, it cannot give less so I don't understand the concept of "over-ride" - to exercise rights under CRA 2015 the owner would need to show they'd taken reasonable care of the caravan, which could include DIY servicing, but that cannot be imposed on the warranty terms.
I pay for 5-years approved servicing to keep the 6-year warranty intact - I regard it as part of the initial cost of buying a caravan - personally I regard the warranty period as the "snagging" time and then hopefully get many years of trouble-free caravanning once the warranty has expired.