Frieghtdogg,
You don't get any 'Time Away from Base' allowance [used for sustinance] or 'Flying Pay' [£10 per hour chock to chock] while sick.
However BALPA have launched a two pronged attack to enforce the Euro Working Time Directive that defines allowances (such as these) as part of your overall pay. Thus, they will still be payable when you are on leave. (Obviously they will be averaged out over the fleet per year etc).
For info: if you join BA as a DEP FO you can expect your variable pay to make up appx 25% (plus / minus a bit) of your gross pay.