I have not invested in such schemes myself.
Small pieces of land without planning permission are worth very little. Let’s say a piece of land is worth £10,000 without planning permission but £250,000 with planning permission. With schemes like this you are being sold the land WITHOUT planning permission – therefore it is worth only £10,000. Companies that say it may get planning permission in the future pump up the price on the basis it MAY get permission at some point in the future. This is indeed true but there are no guarantees it ever will and even if it did, it may not be in your lifetime.
Chances are you are buying an overpriced piece of land. Of course there is nothing wrong with buying land hoping it will get consent in the future, but only if you are buying it at the price it is worth WITHOUT planning permission, otherwise you will make a loss when you sell unless you can find another punter wiling to take the same bet that you did.
As always, there are no free lunches. If there was any indication that planning permission is imminent the price will have already rocketed long before you arrived on the scene.
There are successes, as you would expect. Some of these pieces of land do indeed get planning consent and the companies that offer them can rightfully say that you may make a lot of money in the future if things go your way. To get an idea of the success of the company you are dealing with, ask them how many pieces of land they have sold, have subsequently received planning consent. Due diligence would require that you also check the accuracy of this information – not always an easy thing to do.