Certainly, but I'm puzzled why you'd post a european breaker box question on an american contractor board? The IEEE site might be the better place for exact information. You ask whether it's "legal" to change this breaker, and this question is best answered by someone who knows the laws and codes that exist in the UK. From a safety perspective, changing a regular breaker for an RCD device seems like it would provide an extra measure of safety.
The frame sizes of the existing breaker and the proposed 30 milliamp Residual Current Device (GFCI, type E in america) appear to be physically the same. They should interchange. In North America, it is prohibited to mix and match breakers in a residential panel ("consumer unit" to you) unless the panel is specifically labeled to accept the proposed resplacement breaker ("isolator" to you).
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