Do you have any idea as to how long the flight is as I can assure - its a heck of a long flight & next if anything - God forbid does happen - then who is in the UK to help out & what contacts are available at short notice...ie things could happen after hours & then what??
if was me - then I would say NO & give the reasons....there would have to be a Plan B, Plan C etc for each & every scenario that may arise.
Plus has Grand Ma ever been to the UK before....I work in legal & travel there many times re my work & I can also assure you that it can be a nightmare at the best of times....then next - if anything does happen - then who in the family would take responsibility for making such a foolish decision to allow such a trip as this....remember if something happens - then a long ways from home right & don't forget the time differance re having to make any emergency phone calls.
Not sorry for saying this....but bad idea all round