and to emphasise the point I included a document from their website:
IHTM19120 which confirms that
If the holder of a bill of exchange or of a promissory note either unconditionally renounces their rights in writing, or delivers the instrument to the person liable, this discharges the obligations of the acceptor or promissor, even though no consideration is received. So this is an exception to the rule that a unilateral discharge of a debt requires some form of consideration.