For each of you you can have an estate of up to £325,000 and Inheritance tax does not apply. However this £325,000 limit applies to assets,money and property that you leave to anyone other than the spouse.
SO if your husband were to leave everything to you (and no one else) then you would not have any Inheritance tax to pay as bequeaths to spouses do not attract any Inheritance tax
So if your husband were to pass first and leave everything to you - there would be no Inheritance tax AND he would not have used his Inheritance tax NIL threshold, thereby allowing this to be used by you (along with your own) when you pass.
So IF he leaves everything to you - you would NOT have to sell the London property as you would have no Inheritance tax to pay, but if he were to leave assets worth more than £325,000 to other family - then there would be Inheritance tax to pay - as his estate that has to be considered would be in excess of the NIL band.
I hope that's a little clearer to you
Let me know if I can assist further. I also have added a link here that explains ina little more depth regarding Inheritance tax, the consideration of making gifts prior to death which you may find useful as much for yourself as for your husband.