When I went through the UK, I ended up going alone with no plans, and actually found it to be quite nice. I only ran into problems once, when I couldn’t find any available hostels in Glasgow, so it might be wise to do some looking if there are places you absolutely want to stay, and book those. Otherwise, I highly recommend the no-itinerary thing, as it allows for spontineity – if you don’t like somewhere, move on, or if you fall in love with some place, stay a bit longer. The best way to do it, I say!
I wouldn’t say going on your own is terrible and setting off without a plan is horrible, BUT, what you should do is be familiar with WHAT there is to do, and when you get there, just go with you instincts. But if you go there not knowing what there is to see, you’ll be stopping people every couple of minutes asking what there is to do and to see. I planned the orders of which countries I was going to, but didn’t plan what I would do when I got there.