Well, generally speaking, the more watts, the more miles, but not all things are equal in bikes, as the temperature, weight, aerodynamics, wind speed and rider effort all impact the range you can expect. Look at the max range quoted by the bike manufacturers, and half it. That it probably what you can expect if the conditions you are riding in are not favourable. Also rememeber as the battery ages, it's capacity deteriorates, so range will fall over time too. Will you be easily able to get hold of a new battery in 3-5 years time if it no longer meets your needs? Some brands will make parts available long term, others just want to sell you a new bike.
I appreciate you said you were not keen on the Brompton, may I ask why?
Out of most folding bikes, it has a superior fold, in terms of simplicity, ease and speed. Compare the Brompton to the Btwin you just linked to, that's a mess, but don't take our word for it, go test them both out and compare. Certainly the Brompton is more expensive to buy outright, but that you will find with the Brompton is that the design doesn't change every year meaning you will easily find parts and dealers who will repair and stock parts, and where things have changed, the new parts fit on older bikes as the frame and fold doesn't change. You can bet your last £5 note that
Decathlon will not be offering spares in 3-5 years time for that model and when a part, like a basic hinge fails you might have to scrap a otherwise perfectly good bike. In the long run, owning the Brompton will likely cost you less, as you say you are not the type that swaps a bike often, it should be seen in the context as a longer term investment. The Brompton will certainly win out, and they are incredibly popular that if it didn't work out, it will not lose all it's value and you could sell it without taking a huge financial loss. The Btwin will not be worth very much a few years on for sure!
The Brompton is not a bike many people regret buying (so long as they are serious about folding bikes). Some people have managed to tour the length of the UK on a Brompton, carrying everything they need, so you will not have any issues carrying shopping in rear panniers. You can get a Brompton in 6 speed, adding the motor assist, this will be sufficient for all kinds of terrain, and mechanically speaking, it's a simple contraption, so less faff with maintenance and services. I can't see any reason to look at the competition if you are serious about buying a folder. Lack of long term support for parts from other dealers should be a huge deal breaker and not worth risk on the money saved imo.
If the range is a problem, consider owning one folder that doesn't have a motor for your train rides, and a standard hybrid ebike that will do your shopping without fuss for when you are not commuting. Remember, you can also wear a backpack with an ebike, as you are working less hard to push the bike, you won't get all sweaty (one of the main reasons people like panniers). That said, you need to check maximum weight load of rider and luggage on all your models. If you exceed that, you will have problems with parts failing, and likely void any warranty claims if the manufacturer found out.