Do they have the same rule in Portugal as in France, namely that you cannot register a car that is not a model previously exported to the country by the manufacturer? Weirdly some totally normal cars that have been previously on sale in say the US or Japan, cannot be imported and registered in France, under this rule.
@Avocet1 knows more about all this than I do.
Broadly, the situation is the same across all EU Member states, but there are vast differences with how the rules are enforced.
As has been mentioned, it is illegal to register a passenger car, brand new, in any EU Member State, without some form of approval. For mass-produced cars, that's an EC "type" approval, and for niche vehicles it can be some other lower level of type approval or even an "individual" vehicle approval. The CoC that has been mentioned in other posts, is the certificate that declares compliance with a "type" approval. Individually approved vehicles have a different form of certificate.
However, once registered, the responsibility for compliance passes from the manufacturer to the owner. This is the crucial bit. the point of first registration in an EU Member State, is the critical point at which the manufacturer is no longer responsible for it meeting the rules. This is only reasonable, as the owner could drive it straight out of the showroom to the Halfords round the corner and put different wheels on it, a roof box, a different radio.... etc.
In the UK, at the point of first registration, the type approval regulations cease to apply, and the vehicle is then governed by the Construction & Use regs and the Road Vehicle Lighting Regs instead. These allow more flexibility than the type approval regs, but not MUCH more. What happens is that they are very poorly policed. The same is true in most EU Member States. They will all have their own versions of C&U and the Lighting Regs. As has been said, in France and Germany, these are quite strictly policed, but in the UK they're not. This gives the impression that some mods are legal here, when in fact they're not, it's just that nobody polices them with any enthusiasm.
Insurance companies are something different to add to the mix. They permit some modifications and not others. People think these are legal requirements (and sometimes they are) but often they're not, it's just the insurance company taking the commercial decision as to whether it is willing to still bear the risk (which is why some insurance companies will let you do something and others won't).
Getting back to the OP, I'm not aware of
any UK regulation that says 7 seaters need to have air suspension. (I hope not 'cause I have a 7 seater company car and it's on steel springs)! I'm also pretty certain there won't be a law that says "Landrover Discoveries with 7 seats need to have air suspension". Laws aren't generally written like that, with specific provisions for specific models. Fitting air suspension would, without doubt, invalidate the type approval of a vehicle that was fitted with steel springs (and vice versa). If Landrover type approved with both, there will be no problem swapping, but realistically, Landrover are unlikely to let you look through their type approvals!
Given that the car is (presumably) already registered, we can forget about invalidating type approvals anyway, and we need to start looking at whether there's anything in Construction & Use or the Lighting Regs that prevents it. Again, there won't be anything in any of those regs that specifically says you can't. Instead, the regs will say something like: "Braking systems shall conform to UN ECE Regulation 13H" (or something like that). Reg 13H will then contain requirements that say something like "the ABS / ESC (or whatever) system shall meet the following requirements:" and it will go on to specify a number of tests. So, for example, the ABS system will need to act in sch a way that the vehicle remains stable under a variety of different load conditions and on a variety of different road surfaces when the driver brakes hard and provokes an ABS intervention. The manufacturer will carry out those tests as part of his type approval and they will be witnessed by a government agency. The spec of the tested vehicle will be documented and a stamped-up copy kept by both the manufacturer and the government agency.
So the minute you change anything on the car that alters the specification from what was type approved, you lose your proof that it meets the requirements of ECE Reg 13H which, in turn, means you can no longer provide proof that it meets the C&U requirements. It MIGHT still do so, but you have no proof. At this point, your vehicle would be illegal. This is (one of) the reasons why the Germans are so strict on aftermarket wheels and tyres. The manufacturer will have optimised the ABS pulses to work in harmony with the natural inertia of whatever wheels and tyres the vehicle was type approved with. Change the inertia of the wheel and tyre assembly, and you might (in an extreme case) get something with a natural frequency completely out-of-phase with the ABS pump pulses.
As mentioned, nobody here polices the C&U regs with any great enthusiasm, so you're pretty much free to alter what you like (subject to your insurance company agreeing to it). If the vehicle is then involved in an accident serious enough to warranty and accident investigation, AND the accident investigator is switched-on enough to spot the modification, the legal onus to prove compliance with C&U rests with whoever made the modifications. If you can't prove compliance, you'll be prosecuted (and your insurer will run away claiming that the vehicle had been modified from what he thought he was taking the risk on, so he might not pay out).