About 20% of UK light aircraft fatal accidents occur with a flying instructor on board, but the incidence of fatal accidents during solo student flying in the UK is very low - I can think of two in the last 10 years.
But not all accidents with an instructor on board are ab-initio training - I'd guesstimate a little over half.
So, as a rough figure, about 10%-15% of fatal accidents are during PPL training.
G