There is leeway built into CAP371 already for 7 day duty hours. The maximum that can be planned for pilots is '55hours in any 7 consecutive days, but may be increased to 60 hours, when a rostered duty covering a series of duty periods, once commenced, is subject to unforeseen delays'.
There is no leeway as such in the 14 and 28 day rules, however since most crew will be rostered for standby duties it is often the norm to reduce or remove the planned standby's if duty hours are exceeded. Also, a lot of operators don't fully utilise the fact that early standby's count as 50% for duty hour totals if the crew member is not disturbed during the standby. So whilst some crew might look like they are being rostered to the max duty hours, in reality they are probably some way below the limits.
Realistically luddite there will be a balancing of hours over extended periods. It is legally impossible to roster someone to the limits of duty hours for too many periods. Crew are all restricted to 2,000 hrs a year under EU Working Time Directive which works out at 166.67 hrs per 28 days, taking into account 4 weeks leave per year.