I honestly don't know if the UK Building Regulations (Code) says anything about having more than one main stopcock located inside a single-dwelling property. My guess would be you're not allowed to, or at least strongly recommended not to have more than one main stopcock located inside a single-dwelling...
Regardless I would most definitely want to locate and ensure the existing stopcock is working correctly before doing anything else. If that stopcock fails (original one) for whatever reason and you've installed a second one past the original one, the second one isn't going to do you any good whatsoever...
Not always, but "generally" in the UK the main stopcock inside the house is located somewhere along a straight line with the outside stopcock. I would first look along that line, maybe feel for a loose floorboard.
I know you said you were told the stopcock is "under the floorboards in the dining area", but I would question that. In my (limited) experience, I've never seen the stopcock located in such an area, if it's an old (or original) stopcock it will be under a floorboard somewhere on the ground floor, normally within the kitchen, or the cupboard under the stairs or some other cupboard (coatroom) on the ground floor...
Yes, anyone in the UK can buy a key for an outside stopcock, but as far as I'm aware (unless there has been a change in the law the past few years, since I moved abroad) it is "strictly" illegal to touch that outdoor stopcock. It is owned by and for the use of the water company only. That said, many people (especially plumbers, builders) own the required key and use it - just be aware if you're unlucky enough to get caught doing so you can get in trouble, and if you break or damage the outside stopcock you're liable for its repair/replacement ie Proceed at your own risk, knowing the possible consequences if you get caught using it or break/damage the actual outside stopcock while using it.
Some additional reading that you might find helpful/useful: