Depending on which rental company you are using, the pre-paid fuel option may be the way to go. That is usually only a few cents' difference from street price, and may turn out cheaper, if gas prices suddenly run upward during your stay. (In some cases, you can predict price increases. For instance, gas prices always jump up during the week preceding our Monday Holidays, when people traditionally take driving trips. It usually takes about 2 weeks for prices to come back down again.) MCO is one airport where I usually do take the pre-paid fuel option; topping up on fuel around that airport is really inconvenient if you are coming from the resorts to the south.
The pre-paid fuel option allows you to buy one tank of gasoline (petrol) as part of the base rental charge, prepaid. If you take it, then you should return the car with as close to an empty fuel tank as you can. This should *NOT* be confused with the charge that will be incurred if you don't take the pre-paid option, but return the car with less than a full tank. That is the situation where they will charge upwards of $3 per gallon to top the tank back to FULL.
I know that it is really confusing to be offered all of these options after that endless flight, and I know that the rental companies at MCO really like to push UK tourists very hard to purchase extra options. I've heard them do it, as we usually arrive at about the same time as the overseas flights. Still, the pre-paid fuel option might be worthwhile for you. There is a website which will tell you what the avg. gasoline price is in Orlando at any given time, if you check it right before you leave, you will know if the price you are being offered for the pre-paid fuel option is actually reasonable:
http://www.orlandogasprices.com/
Also, just FYI, most of the gas stations nearest MCO are on the North Exit side of the airport, the side where the locals come and go. If you are coming from WDW via 417 and need to refuel, the best thing to do is refuel before entering the toll road, because otherwise you will have to either pay exit tolls to get off and refuel, or go straight through the airport and out the other side to get to those stations outside the North Exit.