this could be because the underwriter actually has this as a mandatory requirement i.e. that it has to be Thatcham approved, were any of the quotes you got which were lower using Aviva as the underwriter ?
All the quotes were considerably lower saying I have a Thatcham system. Unfortunately, the system will not quote if I say I have nothing installed as my Van is worth more than the 75K limit.
It's just seems a technicallity that they only accept Thatcham when other accept the Motorhome Defender.
At the price MHF are offering it might even be worth removing it and replacing, a shame as it's the best system on the marrket and has just cost me around 700 GBP