s36 Trustee Act 1925 requires only B & C to appoint someone in A’s place. This assumes that the trust instrument does not specifically grant anyone the power to appoint trustees and that A does not have an IIP in the trust fund (I appreciate that you say it is a discretionary trust - sometimes beneficiaries are granted an IIP, revocable or otherwise, out of a discretionary trust).
Life insurance companies can be a bit awkward with assignments of investment bonds. If they will not accept the Deed of Appt/Ret on its own, they may require a Deed of Assignment signed by B, C & D (D signing as attorney for A) to transfer the investment bond to B, C and whomever they appoint as the replacement trustee. I normally include an assignment in the DORA itself to avoid the need for a separate deed and avoid any argument from the life insurance company, but as A is not actually executing the deed, a separate Deed of Assignment may be necessary. I would give the life insurance company a call and ask them whether they will accept the DORA on its own.