No, not halfway across France yet, that's tomorrow. The P38 is plated (certified) to tow a braked trailer up to 3,500 kgs, so as long as the trailer is similarly rated, a P38 can tow it. I mention the rating on the trailer as it depends on different makes. The larger twin axle Brian James and Ifor Williams car transporters are rated for a gross weight of 3,500 kgs and weigh between 750 and 900 kgs empty so will take a P38 and still be legal. Other larger twin axletrailers, Indespension being one of them, look very similar, are the same size (5m long load platform) but are only rated for 2,800 kgs. So adding the unladen weight of the trailer to the weight of a P38 on it and you would be overloaded. So you do need to check the plate on the trailer. If you are overloaded and get stopped by plod, not only do you get stuck with a fine and points but you aren't allowed to continue with the journey until you are within the legal weights. Ordinarily, this would mean offloading part of your load and either leaving it at the side of the road or getting someone else to come out and meet you to take the excess. However, when the load is one big lump (i.e. a P38) you can't really start stripping bits off it to make it lighter.
There's a lot of confusion over what you can and can't tow as the Caravan Club recommend never towing a trailer that weighs more than 80% the weight of the towing vehicle. I suspect a lot of this is down to the affect a crosswind has on a caravan and what that will do to the vehicle towing it. But the important thing is the weights that the vehicle itself has been certified for. You need to check the plate under the bonnet which will give weights. The kerb weight of a 4.6 litre automatic P38, that is ready to go with a full tank of fuel and a 75 kg driver, but nothing else, is 2,220 kgs. But the Maximum Authorised Mass (MAM, what used to called the gross vehicle weight) is 2,780 kgs. So you can have just over half a tonne of crap in it and be legal.
Anyone who passed their driving test after 1999 can only drive a vehicle with an MAM of 3,500 kgs (so something like a big Transit or Sprinter) and can tow a 750 kg unbraked trailer providing the gross train weight (referred to as the combined MAM these days) doesn't exceed 3,500 kgs. So a fully loaded P38 with a fully loaded 750 kg trailer would actually be 30 kgs overweight (2780+750=3530) for someone with post 1999 licence entitlements.
If you passed your test before 1999 you have grandfathers rights so can drive a vehicle with a MAM of up to 7,500 kgs with a trailer of up to 750 kgs giving a combined MAM of 8,250 kgs, OR a vehicle and trailer combination of up to a maximum of 7,500 kgs providing the weight of the trailer doesn't exceed the maximum trailer weight the towing vehicle has been certified for.
So, the answer to a simple question with a long answer is if you passed your driving test after 1999, you can't drive a P38 with a trailer heavier than 750 kgs. This might be something Marty would need to check his licence for as I assume he swapped his NZ licence for a UK one. Dina did the same, having passed her test before 1999 but in Latvia so when she swapped her Latvian licence for a UK licence she was only given the post 1999 entitlements. If you passed your test pre 1999, you can legally drive a fully loaded P38 (2,780 kgs) towing a braked trailer with a combined trailer and load weight of 3,500 kgs making a gross train weight 6,280 kgs.