No experience with those chairs either. It may hold up well and it may not, usually lesser expensive chairs use inexpensive foam which tends to break down. That the reason others have suggested (and I'd suggest) using a separate cushion. Not sure how well a cushion would work with more contoured chair like that one. Using an extra cushion, when it wears out you can just get another.
Not just in relation to chairs but regarding foam in general, I've looked into it quite a bit. Foam is a manufacturers best friend, it's often inexpensive for lower quality stuff and can be deceiving since foam can appear full/padded and yet offer little support at the same time.
It's a popular trend in mattresses and is why newer or more recent mattresses seem to lose support so fast while retaining their shape. Most companies (at least in the u.s.) have policies where the surface of the mattress has to sag a specific amount without any weight on it before they'll consider it a valid issue. Sure it 'fluffs' back up visually but the support is ultimately gone. Another trick is using convoluted foam or eggcrate foam. It appears to have the height but with all the peaks and valleys it reduces the amount of material substantially and breaks down faster. They can take a single foam layer, carve it into egg crate and get twice the height effectively doubling the product.
Not trying to go off on a tangent and mattresses aren't chairs but it may help to understand popular trends and how foam is used in furniture. Many chairs that tend to hold up longer or are used in high traffic areas like waiting rooms, hospital chairs and so on use higher quality materials but are also quite expensive. Likely 350-400gbp or more. In the 75gbp range and lower I wouldn't expect a whole lot of difference in material quality/durability between one model and another.
An addon cushion will be your best bet and you can determine what material it's made out of. A simple plain cushion purchased from a retail store, custom foam where you can pick a different firmness or density (density and firmness don't really relate to one another in terms of poly foam), latex foam etc. Then of course you can consider other things as well such as memory foam (though it may contribute to heat build up and discomfort), shredded memory foam or shredded latex. Many options out there you may not be aware of.