Boiler frost protection

First thing - you need protect the whole system from freezing, not just the boiler. It may be that your boiler is the most vulnerable point in this case, but think for a moment if there are any others too.

I'm presuming this is a condensing boiler (as you wouldn't be allowed to move it otherwise). What I would do is install a frost stat in the outhouse, which brings the heating on. Ideally you want to do this at the lowest boiler temperature setting, but your boiler probably won't have any provision for doing so. What you could do it to wire the frost stat in series with a pipe stat on the return pipe, set to switch off at 30C or similar. That would fire up the boiler for long enough to get some warm water through the system, without wasting much energy. Ideally you want a pipe stat with large hysteresis, so it switches off at, say, 30C, but won't switch back on until it drops to, say, 15C, to prevent unnecessarily frequent boiler cycling.

Assuming you are not intending to heat the outhouse, make sure all the pipework in the outhouse is well insulated (hot and cold), except perhaps around the pipe stat. Might need some provision to prevent the condensate drain and cold water pipes freezing too, such as running them in close proximity to the heating pipework. You may get enough heat from the boiler casing to take the chill off the outhouse, but many modern boilers don't actually lose much heat through the boiler casing anymore.

Some boilers which are intended to be installed in unheated areas have a built-in frost stat, although that usually only switches on the pump, and doesn't necessarily cause the boiler to fire up.