Your question is a little tricky, because a baron of beef in Britain is a lovely (usually large) cut of meat associated with the sirloins. In the US, a baron of beef is an imprecise term used to describe large, less important cuts that are better-suited to braising. And inside round is not a common name for a cut in the US. So we’ll assume that your baron is British and that roasting is the ideal method.
If you cook your 2-pound roast at 325°F (160°C or Gas Mark 3), it should be medium-rare after about an hour. Add 10 minutes if you want medium. In either case, remember to let the meat rest for about 15 minutes after you take it out of the oven before carving.
And our meek and well-intentioned suggestion to you is to eat ground beef for a couple of weeks and buy an instant-read thermometer with the money you save. With that in hand and our guide to roasting beef, you'll never again wonder how to roast meat to the degree you want.