I didn't know you could feed an e-Werk DC. That could be handy...
What you certainly shouldn't do is connect a solar panel to a phone/tablet/kindle/etc directly. They're expecting regulated 5V with fairly tight tolerances, and exceeding this is likely to cause the magic smoke to escape.
You're on the right track with using a buck/boost regulator to bring the panel's output within spec, but it's never quite that simple. If you're charging a battery, there's a balancing act involved in keeping the voltage/current at the sweet spot to extract the maximum power from a panel. If you're powering a USB device, you don't have that luxury - it expects 5V and (dumb charger identification issues aside) will draw whatever current it wants to draw, and if the source can't supply that much power the voltage will drop out. If you're lucky you get intermittent charging. If you're unlucky the device goes into a sulk and refuses to charge. If you're really unlucky the device wakes up and starts consuming power from its internal battery faster than it's drawing from the USB port, and you end up with a net discharge.
Given that you're asking the question, I'd suggest what you need is a commercial solar charging product that provides you with a regulated 5V USB output. It'll either have to be large enough that it can comfortably exceed the current demands of whatever device you intend to power, or work by charging a battery with whatever power is available at the time, which can then be used to charge your device at the current it chooses.
If mobility isn't an issue, then an alternative approach might be to use a suitable panel to charge a deep-cycle lead acid battery[1], and then a buck regulator to efficiently step that down to USB spec. There are no shortage of off-the-shelf panels designed for charging car/boat/caravan batteries, and a decent-quality (something rated for iPads) car USB charger should be based on a reasonably efficient regulator.
If you want a panel to work usefully in the UK, take the power rating of the device you want to supply and double it. So a typical smartphone-like device charging at 1A is going to need a panel in the 10Watt region. Compare the area of a 10W panel with that of a Powermonkey type device and you can see why "personal scale solar" is usually disappointing.
[1] They're heavy, but they're cheap and extremely non-fussy about how they're charged. Gel cells are a bit more portable than wet ones.
