I recently had to deal with the estate of a family member who died in the UK. There was a large inheritance tax bill to pay and it got me thinking about my own estate, although I am still young. At the same time I'm thinking of having kids in Thailand. It turns out that even if I never set foot in the UK again and the kids grew up entirely in Thailand, the chances are they would be liable to pay UK inheritance tax on my estate, including any Thai assets (well technically it's the estate that pays the tax, but it works out the same). I asked the lawyer if/how this could be avoided and he said that HMRC (the UK tax authority) would need to be convinced that you had decided to base yourself somewhere else permanently, and that that might be difficult to do if you didn't have the right to stay there permanently. Just being non-resident at the time of death is not enough. If you are on yearly extensions, his view is that's a grey area and you could easily end up in a dispute. On the one hand you can realistically expect to be able to go on extending indefinitely, but on the other you don't have a right to extend, it's politically turbulent, etc. If you have PR there shouldn't be a problem and the estate will be subject to Thai inheritance tax, which is a lot less and seems more appropriate if I have lived most of my life in Thailand and the kids / beneficiaries are Thai.
Another way to say it would be that you need to convince the UK authorities that you have emigrated and it's hard to do that if you are on a non-immigrant visa.