3 sciences means 1 extra block for the single qualifications, rather than being taught all 3 for combined science. Takes up 2 blocks for combined.
English + English Lit - no extra block.
RE - as everybody has to do it to some extent, might as well get a qualification out of it - no extra block.
Maths + Further Maths or Stats - offered to the highest set in Maths - no extra block, as they do it in their usual lessons.
Leaves 2 blocks for real choices for the top sets - (Geography/History/Citizenship/Sociology/Psychology/Business/Computing/Art/Drama/PE/Food Prep & Nutrition/DT/Music/AAQ in Health/Sport/MFL/etc)
Or three blocks for the others who are taking combined science instead.
Add in the ones who need extra Maths/English/EAL support/SEND support who are more likely to have singleton allocations on their timetables to fit their needs, kids who are going to Greek/German/Polish/whatever school on Saturdays sitting a GCSE in their community language (quite possibly sitting the exam at the school for their convenience) and a handful doing something like Music grades, LAMDA (although they may take this as a second qualification if they've picked music or drama as an option) and you've got a pretty broad base for qualifications, whether they do 1 extra block of science + computing or Art + Drama or Music and PE.
When they have just 2 or 3 choices, it's not fair to demand that they take something they don't want to - MFL has been on the receiving end of kids voting with their feet/pens and refusing to work until they're allowed to drop it or they're just not entered for years - and the poor grades for those who haven't been able to get out of the lessons to do something else reflect it.