Equivalent Mattress to F&B Fitzgerald Mattress

Good evening,
Could you please advise what your closest equivalent to Feather and Black's Fitzgerald Mattress at tension 6 is please? We would like to upgrade from a King size to SuperKing size mattress and generally happy with the feel of the mattress that we have currently (which is the Fitzgerald King Tension 6) but looking to see what other alternatives/equivalents are available.
Kind regards,
Louise.