Hi there, I hope I can help.
The MAP is licensed for up to 72 hours after unprotected sex. It is taken as a one-off dose, ideally as soon as possible after sex. The most commonly prescribed brand is called levonelle if you want to google it.
As for effectiveness, the key is to take it ASAP. Of course it is not perfect, and there is a failure rate. The figures I quote when prescribing for success (i.e. preventing a pregnancy) are 97% when taken within 24 hrs, reducing to 67% up to 72 hrs.
An emergency coil can be used to prevent pregnancy up to 5 days after sex. This can be arranged at a FPC.
As with every contraceptive method, there is no guarantee that it will work. The sooner it is taken, the better. If it were to fail, then the hormones will not harm the developing foetus.
To get hold of the MAP today, you can buy it over the counter at some pharmacies (costs about 20 quid), some A&E departments, out of hours GP. NHS direct should be able to help you with availability for your area.