If you are going to Cuba, the first thing to check is whether your TO or airline has included the cost of your tourist card/visa in the price they are charging - most do because they know you have to have it and they know the hassle involved in obtaining it! If it is included, they'll post it to you a couple of weeks before departure.
If it isn't included then this is one of the few instances where it is often cheaper and definitely easier to use one of the on-line visa service companies rather than dealing direct with the Cuban consulate. The card itself only costs £17 if you can go to the Consulate in London to collect it yourself but if you need to do it by post the additional admin fee means that it will cost £39 plus the cost of whatever your bank charges for a bankers' draft because they won't accept personal cheques or card payments. You can buy on-line from one of the commercial visa agencies from £27 if you shop around and apply in good time before you go. Either way what you'll receive is simply a blank card perforated down the middle and you fill in all the details yourself - you don't have to send off your passport to get it. The reality is that it simply confirms that you have paid what is in effect just another entry tax. And if the worst comes to the worst you can usually obtain one in your departure airport in the UK. The check-in staff will usually ask to see it to confirm that you have filled it in correctly and if you haven't they'll direct you to an on-site travel agency that will sell you another.
Finally, when you go through Cuban immigration on arrival they'll check it and give you half back. Keep this somewhere safe and remember to keep it handy because you'll have to produce it when you leave Cuba - if you can't produce it they'll charge you for another one! Also on arrival make sure you have a copy of your travel/health insurance handy as they ask to see it and will sell you one if you can't - which will probably cost more than you could have got at home. And don't forget to keep 25CUCs handy for paying your departure tax at the airport - you won't get into the Departure lounge without the receipt.