(xxi) “Purchaser” means a purchaser in good faith for valuable consideration and includes a lessee, mortgagee or other person who for valuable consideration acquires an interest in property except that in Part I of this Act and elsewhere where so expressly provided “purchaser” only means a person who acquires an interest in or charge on property for money or money’s worth; and in reference to a legal estate includes a chargee by way of legal mortgage; and where the context so requires “purchaser” includes an intending purchaser; “purchase” has a meaning corresponding with that of “purchaser”; and “valuable consideration” includes marriage [F6, and formation of a civil partnership,] but does not include a nominal consideration in money;
I think that the key to understanding that sentence is understanding what exactly, nominal consideration is less secure than. In context, it appears that nominal consideration is less secured in terms of contract enforceability than paying consideration that is more than nominal and instead constitutes substantially equivalent value to what is received in the transaction.
In the U.S., we would call a contract made for far less than substantially equivalent value that prejudices a third party interest a "fraudulent transfer".
It does not appear to me that this statement is saying that nominal consideration that amounts to an extreme bargain sale is less secure or enforceable than, for example, an outright gift.