Have you thought about the child maintenance you will be paying for your daughter? To put it simply, the more you earn, the more you will pay so you could be in the situation where you are earning great money in London but keeping very little for yourself after the high bills and cost of living. You could end up no better off in London than you would be in Belfast and lose contact with your daughter. Long distance relationships with your children are difficult especially if you only see the kids two or three times a year.
You need to prioritise. Get a job in Belfast, pay your bills and make sure you eat well. With your health problems that is paramount. You won't have a lot left over for luxuries or non-essentials but it's been said on this thread already. Being involved in the day to day life of your daughter is the greatest gift you can give her. As well as the routine stuff that has to be done - school runs, homework, chores - there will be time for fun and 'togetherness' which need cost nothing or very little.
She has had a lot of disruption in her life and you can give her the stability she needs, just by being there for her.