Sealion 7 is a challenger for the IM6 and has been used in group tests I've seen.The BYD Sealion 7 is probably closest in terms of dimensions (4,830mm long, 2,930mm wheelbase), but the boot space is 520L with seats up / 1,789L with seats folded, and the Frunk is only 58L. The Sealion 7 has an RRP of £47k-59k too, so for a "challenger" brand, it's quite pricey compared to the MG.
If we are just going on size and space and not price we could say the S6 is challenger for the im6