How do I answer "What is the total amount in GBP?" on the UK visa form?

You figure out the answers in the other order. First, work out the total cost of your trip: flights, hotels, meals, train tickets, everything, in the currencies you're paying in. Then convert to pounds and calculate the total. This is #2.

Next, establish what someone else is paying -- did they buy your plane ticket? Will they be paying for the hotel stay? Are they reimbursing you for what you spend? Convert all that to pounds and add it up. This is #1.

In the end you can say something like "my employer [or my uncle or whatever] is covering X pounds of the Y pounds this trip costs." For some trips, when someone is covering all your expenses, the numbers will be the same: just put the same number for both. For some trips, when you are covering everything, X will be zero, so put zero for it. For most, the numbers will be different. Just work it out and convert to pounds and tell the truth.