I think if a company showed interest you would first need them to sign a contract acknowledging your 'intellectual property rights" before giving them all the information of your invention. Also file a detailed copy of your invention with your Solicitor or bank, dated and sealed. Realistically though, even if you have taken out a Patent on your invention, if a big company did steal it, it would cost you a lot of money in legal fees to take them to court and challenge them. If it is something you could get a prototype (original model) made up yourself, that gives you more leverage. Hopefully a Lawyer will give you some advice on this.