sorry to dissagree, but working as a subbie to a national plumbing company, i have seen burst cold supply pipes in attics on insurance call outs literally dozens of times over the past 20 yrs.
always the same thing, loft is lagged well, but some how a section or full run of pipework feeding headers or taking an up and over shortcut for a shower supply has not been lagged at all and is laid over the top of loft insulation..
it WILL freeze if sufficiently cold, especially if water is standing in pipe for any time,will expand and split the copper or crack the plastic, and will then flood everyting below it when the ice plug thaws.
the only sure fire way to stop attic freezing is to get up there and lag all supply/feed pipes with decent thickness pipe lagging and header tanks with a bylaw kit at least.
what is worth remembering, is if loft insulation is correct and doing its job, the heat produced by the heating system WILL BE INSULATED AND KEPT BELOW THE ATTIC FLOOR so everything above, ie the attic space, will become chilly freezing!!
the only pipes that the central heating can protect are those within the direct area of heated dwelling space.
regards,
mark (down in not so sunny cornwall-darned global warming!!)