The answer to this question depends upon the type of joint ownership of the house. The couple might either be 'joint tenants' or 'tenants in common':
http://www.firstrungnow.com/joint-ownership/jo int-tenancy-tenants-in-common.aspx
If they are joint tenants, the surviving spouse would automatically take control over the full value of the house. (That would happen even if there was a will to the contrary. A joint tenant can't leave 'their share' of a property to anyone other than the other joint tenant). The residual estate would then only be worth a maximum of �25,000 and surviving spouse would automatically inherit it all.
If they are tenants in common, the surviving spouse only automatically keeps 'their' half of the house's value (i.e. �62,500). However the estate of the deceased person would then only be worth a maximum of �87,500 (assuming that he/she was the owner of everything except the house), so it would still be below the �125,000 limit and the surviving spouse would still inherit everything.
However, if the value of the house was to rise significantly prior to the death of the first partner, there would come a point (under tenancy in common) where the children would start to inherit part of the estate. (Only the son would inherit anything if the husband died first, but all 3 children would inherit equally if the wife died first). This would not occur if there was a joint tenancy.
Chris