You can read about my startup at www.stugreen.com
The answer also depends on what you define as "startup".
From the business's point of view the best thing is to have your whole time, attention, and commitment. Most businesses do not benefit from their founders' attention being divided (on the contrary, they suffer critically from this).
But from a personal point of view, having your fingers in many pies is clearly beneficial, especially before you know which of your businesses will "stick".
I think the answer to this is changing as the tools available become better at allowing us to be very effective with short bursts of work, but what's certainly correct is that you should not say no to opportunities - but be conscious of how much you can contribute to each and what it's costing you in terms of work not done on others.
Where this gets difficult is being able to tell the difference between a failure that isn't worth more effort and a failure that needs more work to become a success. ;)
looking at your website gives me more convidence in your person then looking at your HN question. You seem to be veeery unsure about the way you want to walk the next months. From my experience the best way to find out is to follow people who do great what you want to do. That means reading blogs, HN but also working together with people you admire. Maybe you can find some great people on the net or around you in the UK to work with.
Btw. I really like the design of your websites! Did you do it yourself?
So was just curious as to what people thought. Yes I designed it all myself. Had a guy do the Halogy logo and Project Bubble logo though.
Note, if you can get something started, (possibly also funded,) and operating largely without your intercession, then iterating on that model quickly can be completely awesome. But get something going first and don't try twelve things simultaneously - been burned by that one myself.