Thank you posting here and for being able to reach out for help. Welcome to the forum. Please accept my condolences on the loss of your mom and dad. Like many older people here, both my parents have died. Each time I was in Australia and it was so difficult. I have some understanding of the sadness in your life.
Be happy that you did see mom for a brief time before she passed away. I know having long distance family is hard and returning frequently to the UK is expensive. So you did what you could and I am sure you kept in touch. It doesn't help your grief much I know, but consider this, you would still be grieving even if you had lived next door to mom. So please do not give yourself a hard time with guilt.
I know stories from others about loss are not always helpful so I will keep this brief. My dad died before I could afford a trip to the UK and mom died several years after my last trip. Yes I felt guilty because I knew my dad was dying and could not go. Although I did not know how seriously ill my mom was I did not rush off to see her. She always seemed to get well again.
So lots of grief and guilt. I did what you are doing, sit in my home and cry. At work I had to fight to stop crying. Please be reassured your mom loved you and you managed a last short time with her. That's all that matters. Your overwhelming grief will pass and settle into sadness when you remember you will not see her again. We can tell ourselves all sorts of logical things but it does not help grief.
In the end we must sit it out and wait for our grief to settle to a gentle mourning. It will happen I know and I want you to hang on to this thought, that gut wrenching pain will slowly subside. Your guilt I think is a product of your sadness. Let the guilt go, it was not your fault you did not see your mom earlier because no one knows the time when they will pass away. Keep the best memories.
Have you thought of holding a memorial service for mom and dad? Not necessarily a formal service in a church or other venue but perhaps a gathering at your home. It will give you a time to mourn with others who knew your mom and dad. Get out the photos and talk their lives. You may cry but it is a huge help to be able to talk about your parents in this way.
I hope all goes well.