Best mattress for two different sized people

I am looking for a decent quality king sized mattress for less than £1000, ideally a good chunk less! For the first year or two it will be used by my husband and I but after that it will become a guest bed, when we plan to buy ourselves a new bed on a slightly higher budget. We are quite different sizes: my husband is 6'2" and weighs 85kg and I am 5'3" and weigh 56kg. Bearing in mind that it will become a guest bed, is there a single mattress type which would suit both of us? The base is slatted and we prefer the firmer end of medium level of support.