The Sinclair QL has not (at least not very much besides the general cost-savy technical design and the Microdrives) been influenced a lot by ZX Spectrum computers - It's rather the other way round: ZX Spectrum Plus and Toastrack did pick up the Dickinson design of the QL computer (For those not familiar with the timeline: The QL was the first Sinclair computer that used that "new", blocky design - Both Spectrum Plus and Spectrum 128 came after the QL).
The QL introduced a "Standard line editor" that understands a lot more Cursor key combinations than just "delete" (<CTRL>Right) and "backspace" (<CTRL>Left). There's word-wise (<Shift>+Cursor), line-wise (<ALT>+Cursor) and page-wise (<Shift><ALT>+Cursor) forward and backward movement and deletion (with added <CTRL>) key combos defined in the line editor implemented by QDOS, so it appears somewhat logical that engineers didn't design in an extra key for just one of many operations.
<Cursor>+<CTRL> deletes a character
add <Shift> and it deletes a word
add <ALT> will delete a line and
add both <Shift> and <ALT> will delete a page.