Antennaes are the easiest way to tell crickets and grasshoppers apart. Crickets have long, whip-like antennae that are longer than the cricket’s body. Grasshoppers have stouter and shorter antennae.
There are 14 species of crickets native to Britain, plus some alien established species. There’s an article on the Speckled Bush Cricket in RHS, The Garden, November 2012, p.33.