I was told that typically (as in most of UK for a few cold nights with reheat during day) it was the localised cooling effect of cold draughts blowing past that are most likely to lead to frozen pipes in loft or under floor than simply falling temperature. SOme areas will be more susceptible of course. It'd be unusual to to have totally unlagged pipes under floor, so I wouldn't overly worry about it. If you can actually crawl around underneath then make sure everything has at least some covering.
If I was worried and the house was occupied I'd set the HR92 downstairs to 8-10C, and expect that would fire boiler and circulation before harm was done. Or maybe a short blast to 20C at 3am on all the end of pipe run rads. If house was going to be unoccupied I'd remove the heads and open manually, so that circulation could pass by when boiler and pump fired.
Even though you have three BDR can you not still connect frost stat to CH valve "as normal" and use its orange wire as an alternate / parallel boiler and pump trigger?
If it is one particular area of crawl space troubling you rather than lots of branches on the pipes then you could maybe put a zone valve across the far end flow and return there (ie probably just under the furthest radiator). Bring power to it over 3+E cable via a nearby frost stat, connecting core 3 to the orange wire on that valve to run back and trigger the CH valve (which in turn can trigger boiler and pump).