can I insist it is either sold or considered as part of their share if they wish to keep it?
Short answer - yes
I would agree with Lizzt2007. Unless the will says something like you are each to inherit one third of each individual property then you each inherit one third of the value of the entire estate rather than particular properties. It is then down to the executors to decide how to achieve that.
You mentioned "death duties". Can I assume that your parents have a will leaving everything to each other and then, when the surviving spouse dies, everything is left to the children?
In that situation, your parents can leave you and your siblings up to £1 million without paying any inheritance tax. Then inheritance tax is only payable on the value of the estate above £1 million.
Here is an example of how it would work.
Let's say the UK house is worth £1 million and the overseas house is worth £400k. Also, to keep things simple there are no other assets (bank accounts, investments etc) to consider.
So the total value of the estate is £1.4 million. In the situation of two parents leaving everything to their children or grandchildren including the family home then the first £1 million is exempt and £160k inheritance tax is payable on the remaining £400k (tax rate is 40%).
Unless your late parents had £160k lying around in cash/investments then you would need to sell one of the properties to pay it (I guess that it would be the UK home?)
So the net value of the estate after paying tax is £1.24 million. Each sibling then inherits £413k.
If your two siblings wish to keep the overseas property worth £400k then they would likely each take an equal share of it (so that is £200k each) and then take the remainder of their inheritance (£213k) in cash from the sale of the UK property.
Meanwhile, since you have no interest in the overseas property you take the full £413k in cash.
Hope this makes sense.