roast in the bag kits

I agree with phizzimum - read the ingredients and you'll see the mix is basically spices, sometimes herbs and S&P. You may already have some or most of these anyway. Any other ingredients listed you probably don't (or won't) need; many of them contain E-numbers too. Example: potato starch is in there as a thickener for the sauce, but you can use cornflour or just flour. The ingredients list will start with whatever there's most of and the last one will be the least, IYSWIM.

The bags, though convenient, aren't essential; you'd get the same effect by putting the meat into a casserole dish or tin and covering with the lid or foil.

Part of the fun of learning how to "make your own" is experimenting until you get the taste you like. Make a note of what you've used and, once you've cracked it, you can make up your own mix in bigger portions and keep it in a screw-top jar in the cupboard. Just as convenient, you have control over what goes in and, best of all, pennies saved.