There's not usually a specific pool of staff for that, but rest days will be built into the link and naturally covered by other staff elsewhere in the link - so no problem there. There will also be a number of 'spare' turns built into the link, which provides the contingency for sickness, annual leave and so on. That way, at least, they're only at risk of being messed about a bit when they're booked spare.At the risk of asking a question that's probably come up before, why don't TOCs develop a pool of staff who are purely employed to cover rest days and other contingencies like staff sickness and holidays?
Contrast that to the situation in the signalling grades, where there are usually general purpose relief staff to cover sickness, annual leave and so on, who are booked spare for the majority (or all) of the time apart from rest days - but receive a fairly substantial flexibility premium for their trouble. Some areas still, if I'm not mistaken, have rest day relief staff purely to cover, as the name suggests, residents' rest days - in our area, though, as with traincrew, rest days are built into the base roster so that most, or all, turns are covered by a resident somewhere on the base roster.