A hypothetical WE.177D would follow the existing A, B and C variants. The expectation is that Britain would improve what it already had because it was not developing a wholly new system at that point. The D designation describes a possible follow-on in this discussion, rather than an identified weapon that entered service.
Notable Point · 56:52-57:15
Why a hypothetical WE.177D would extend an existing family
A possible WE.177D is envisaged as an improvement to the existing weapon family, consistent with Britain's limited development programme.