No, it's not illegal - just bear in mind that some insurance companies won't recognise non-vet jabs, and some policies won't pay out if your dog isn't vet jabbed. Also, kennels won't accept non certificate jabs. If neither of those things are likely to apply to you - crack on. I get mine done for £15 a dog at the local pet shop ( Pets at Home ) - for £15 a year, I know it is a proper, in date vacc that will be signed onto the vacc cert by a qualified vet. Personal choice.