That is a good amount for a child that age - mum is ungrateful.
Lego from market place - buy loose, wash, and put together into a shape, download a few free models to make. If he does not have a container, buy a biggish wooden box and tell him you will fill it eventually. Others family members may offer more.
If he is the type to read comics, a short magazine subscription e.g https://www.natgeokids.com/uk/subscribeuk
Does he have any kind of electronic device - £25 could buy him a new game/cartridge esp if you have a local shop that sells recycled ones, but Smyths have a number of cheaper ones suitable for his age group.
Is he in to figures? They are expensive, but he will remember a favourite one. Pokemon, He-man, for example. Spiderman is a perennial favourite (bought for my oldest GS who is 22, and youngest who just turned 6)
If he is the cuddly sort he may love a squishy.
But as it is from you he will be happy (I fondly remember as a still-at-school teenager buying a nephew a plastic sword, armour and helmet for a few pounds and getting a huge supermarket box to wrap it in - his parents brought him an expensive Scalextric . He did not even open it, and spent all Christmas day either sitting in the box playing racing cars or playfighting with anyone who would join in.)
https://www.mumsnet.com/talk/christmas/4914662-ideas-for-7-yo-boy
https://www.mumsnet.com/talk/ninety_days_only/4945722-gift-ideas-for-8yo-nephew