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I don't think putting it as a 4th science is a good idea because it may cause problems at later stages.
I believe most universities would not consider a Computer Science/ Computing A level as a "science or maths subject" for admission onto a science degree as I don't think it will teach what they want science subjects to teach. Geography and Psychology A levels are included because they teach students a scientific way of thinking, familiarise them with the idea of scientific papers and citing references and usually include a statistics component. I am happy to be corrected if this assumption is wrong, however.
I think CS may well be useful to many students, and should be valued as an option, but I don't think it is comparable to Biology/Chemistry/Physics, as I don't think it teaches the same skills. I also think gaps in scientific knowledge can be problematic if you go on to study science at a higher level, and I think it's wrong for children to be shutting doors at the age of 14 or possibly younger.
I agree this could actually be a route to teaching less science. Someone with two Cs in Physics and CS won't have a broad knowledge of the sciences. In some ways, this may devalue CS as an option.