Hi Pumpkinhead. Sorry you're having to deal with such a painful decision. I also live abroad, a long way from the UK, and my mother died last year. Our situation was a little different in that we didn't know how long she would live for. There were times when they told us she might not last through the night, and other times we thought she could live for another few years. It's very very painful being torn between the needs of your mother and the needs of your children (and of course your need to be with her).
I went back alone for a short visit when she had her accident, which I was very glad I did, as it was the last time that I saw her fully conscious. But it meant leaving my 3-year-old and 5-year-old, which was awful too. I then went back again with just my 3-year-old when she was deteriorating, and was actually with her when she died. I was so so grateful for that. Then my DH flew with my 5-year-old for the funeral.
I can't tell you what to do, but I feel that the financial aspect is the least important thing. Do you have somewhere to stay in the UK? Another thing I'm sure you will have considered is that you won't be able to fly in the later stages of your pregnancy, so if you go you should go earlier rather than later. How old are your children? Are they at school? How long could your DH take off work to fly to the UK when you have the baby? Is there anyone, anyone at all, who could help you with the kids in the UK in this emergency situation? People are often very keen to help when they know someone needs it, they don't like feeling helpless, they would rather be able to do something practical to help.
Also, can I gently point this out? You say 'if the worst happens' - but you also said your mum has been given 6 months. So it sounds as if you're not really accepting the truth either. I know it's very hard, it all felt so unreal for me being thousands of miles away, on the phone to my sister at the hospital. And life goes on normally around you, so it feels like some awful dream. Part of me still feels guilty that I was abroad when she had her accident and not there when she needed me most. But the reality is there is not much I could have done. And I couldn't take my children out of school and find a house to live in when it was for an indefinite time, and I couldn't have given her much time and attention when I also had the kids to look after.
Only you can know what's right for you. But I will say that I was very 'lucky', though it feels a strange way of putting it, that I was with my mum when she died. I would certainly have always regretted it if I had not seen her.