First, you should be careful buying an AC/AC adapter from the US and use it in the UK. The mains here are 110V, and I believe in the UK it’s 230V. If you use the rain bird transformer (110V -> 24V AC) in the UK, it will probably output 48V AC, which will destroy OpenSprinkler in no time.
It’s possible to modify OpenSprinkler to use DC power instead. The power conversion circuit works with DC as is, so no need to change that. However, the triacs need to be replaced by transistors or mosfets in order to interface with DC devices. Flyback diodes should also be added to interface with sprinkler solenoids. On the microcontroller-based OpenSprinkler 2.0, the PCB design has considered these potential modifications (including PCB holes for both transistors/mosfets and flyback diodes). But OpenSprinkler Pi doesn’t have these yet.
The main reason *not* to go with DC is that most sprinkler solenoids are designed to work with AC power. So all standard sprinkler controllers are also powered by AC adapter. Since we don’t make custom sprinkler solenoids, we have to stick to the industry standard. While you can probably operate them with DC as well, there is a liability issue if things go wrong.
So in the end, we can leave these options available, but you have to make your own modifications if you want it to work with DC, well, unless if you can convince me that in the UK market, most sprinkler solenoids operate on DC power 🙂