The E484K has indeed arrived on the background of B.1.1.7 and detected thanks to @CovidGenomicsUK. This figure shows number of sequences deposited over time. There may be independent acquisitions which is of concern but not surprising given B.1.351 and P1 both have it.
We generated 'pseudo' viruses bearing the Spike protein of SARS-CoV-2 that had all 8 B.1.1.7 mutations + E484K. We mixed virus with sera from vaccinated individuals (Pfizer) Overall, E484K substantially increases the amount of serum antibody needed to prevent infection of cells.
What does this mean? well the number of sequences is low at present, though enhanced surveillance is being undertaken by @PHE_uk. There may be more cases out there given how high transmission has been. We need to continue vaccinating and drive down transmission.