It's been a number of years since I did Britain, but I just used trains for long legs and buses for short ones. I don't know, maybe go online and check some sample bus fares (I pulled up a bunch of companies with a google search of "bus fare Britain"). Add these up to kind of get an average and see how much extra you're paying (or if you save anything) with haggis.
I guess the thing I like about traveling independently is that if you like a city and want to stay a few days extra, you can. Or if once you're there, and find out about all kinds of cool castles, or hikes, or hot babes or whatever, you can hang out longer. If you're on a structured tour -- as opposed to a "hop on, hop off" service -- then you are missing out.
But it ultimately depends on the type of travel YOU feel comfortable with!
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"Life is not measured in the number of breaths we take, but by the places that take our breath away..."