The use of the character "!" in BCPL is much later than the development of the B language from BCPL, in 1969.

The asterisk had 3 uses in BCPL, as the multiplication operator, as a marker for the opening bracket in array indexing, to compensate for the lack of different kinds of brackets for function evaluation and for array indexing, and as the escape character in character strings. For the last use the asterisk has been replaced by the backslash in C.

There was indeed a prefix indirection operator in BCPL, but it did not use any special character, because the available character set did not have any unused characters.

The BCPL parser was separate from the lexer, and it was possible for the end users to modify the lexer, in order to assign any locally available characters to the syntactic tokens.

So if a user had appropriate characters, they could have been assigned to indirection and address-of, but otherwise they were just written RV and LV, for right-hand-side value and left-hand-side value.

It is not known whether Ken Thompson had modified the BCPL lexer for his PDP computer, to use some special characters for operators like RV and LV.

In any case, he could not have used asterisk for indirection, because that would have conflicted with its other uses.

The use of asterisk for indirection in B became possible only after Ken Thompson has made many other changes and simplifications in comparison with BCPL, removing any parsing conflicts.

You are right that BCPL already had prefix operators for indirection and address-of, which was different from how this had been handled in CPL, but Martin Richards did not seem to have any reason for this choice and in BCPL this was a less obvious mistake, because it did not have structures.

On the other hand, Ken Thompson did want to have "*" as prefix, after introducing his increment and decrement operators, in order to need no parentheses for pre- and post-incrementation or decrementation of pointers, in the context where postfix operators were defined as having higher precedence than prefix.

Also in his case this was not yet an obvious mistake, because he had no structures and the programs written in B at that time did not use any complex data structures that would need correspondingly complex addressing expressions.

Only years later it became apparent that this was a bad choice, while the earlier choice of N. Wirth in Euler (January 1966; the first high-level language that handled pointers explicitly, with indirection and address-of operators) had been the right one. The high-level languages that had "references" before 1966 (the term "pointer" has been introduced in IBM PL/I, in July 1966), e.g. CPL and FORTRAN IV, handled them only implicitly.

Decades later, complex data structures became common while the manual optimization of incrementing/decrementing explicitly pointers for addressing arrays became a way of writing inefficient programs, which prevent the compiler from optimizing correctly the array accessing for the target CPU.

So the choice of Ken Thompson can be justified in its context from 1969, but in hindsight it has definitely been a very bad choice.