A quick check online for the necessary data is not wholly conclusive at the first attempt, but it is at least usefully indicative.
I am fairly sure that most GP14 combi-trailers use a 250 road base, but I cannot quickly find any weight data for that size.
I would allow 214 kg for a fully down-to-racing-weight GP14 complete with mast and launching trolley, with all removable kit (rudder, anchor, oars or paddles etc.) carried in the car. Minimum hull mass is 133 kg; minimum mass of a metal mast is not specified in the Class Rules but that for a wooden mast is (at approx. 11 kg), so allow about the same for a metal mast. Mass of launching trolley from this manufacturer, https://boattrailers.co.uk/Product/0000002175/250_Standard_Launching_trailer) appears to be 70 kg. Hence total 214 kg, minimum. Then the mass of the empty roadbase needs to be added to this.
But if the boat is not down to minimum weight then make your own assessment of how much you are over; or alternatively check it at your local weighbridge before you commit yourself to the change of car.
What is easier to find, at least from this trailer manufacturer, is the maximum gross mass of the laden trailer. That is an alternative and very useful guide, always assuming that your boat is not overloading the trailer of course …
At https://boattrailers.co.uk/Multicraft_Roadbase they give the gross capacity of their 220 roadbase (slightly too small) as 350 Kg, and that of their 275 roadbase (slightly larger than I would expect you to use) as 400 kg. The 250 roadbase which I would expect for a GP14 would presumably be somewhere between these two figures, but even the higher figure is still (modestly) within the towing capacity for your proposed new car.
Their stated payload for the 220 roadbase is stated as 200 kg, which on the basis of the above calculation is just a little too small; they don’t state the payloads for their larger roadbases. But 200 kg is only very slightly under-size, which suggests that the 250 is likely to be the correct one. Since the 275 roadbase appears to be larger than you need, and its gross capacity is still within your limit, it would seem that you should be alright.
However if you wanted to be doubly certain you could look at the data plate on your own trailer. Failing that you could check online for the data from your trailer manufacturer, or alternatively contact them and ask them. Again this is always assuming, in either case, that your particular boat is not overloading your particular trailer.
The above checks are indicative only, as to what to expect. But the only way to physically verify what your particular boat actually weighs when on her trailer is to weigh it, at your local weighbridge or otherwise. Let’s just hope that you don’t find that it is over the capacity of your existing trailer …
Oliver