Hello Dusty,
If you need the extra capacity then go ahead, otherwise it makes the caravan non standard and might affect its resale/trade-in value. You would have to ask Bailey to get their specific reasons for downrating the MTPLM on the Senator, but it seems to be quite common, and one of the possible reasons is to do with the choices of towing vehicle a particular target cohort of customers are likely to use. We know (because there have been protracted debates about it) the (miss-guided) perceived importance that some caravanners place on getting the exact towing ratio, and if its a percent over they will refuse to tow it, well if your original MTPLM of 1670kg represented 85% then the towcar's Kerbweight would have to be 1965Kg minimum. Now with a raised MTPLM to 1800Kg to keep to the 85% figure you would need a towcar of 2118Kg kerbweight. Not too many family saloons tip the scales at that, so fewer people might choose the Senator. There was also the restriction of only cat B licence holders where the combined MAMs must not exceed 3500Kg, and the ratio not exceeding 100%. At 1670kg this caravan could be towed by a vehicle of MAM of 1830Kg. Its tight but it leaves the car with only maximum 160Kg payload. But at MTPLM 1800 the tow car could only be 1700MAM, and that immediately fails the 100% ratio criteria as the MAM is always more than the kerb weight. There is a slightly wider scope than I have indicated because I haven't taken into account the effect of the way the caravans nose weight is distributed.