Self referral to NHS abortion services or the charitable ones like Marie Stopes or BPAS (with the costs to the woman covered by the NHS) means that you can go straight to the specialist service and start talking to an informed person who can talk you through all the options (including continuing with the pregnancy). Usually this is someone with counselling skills too and the specialist service can offer emotional support and separate counselling sessions as needed.
While you could have only a very basic chat about this with a GP it would be very rushed, and the GP would be referring the woman on to the specialist provider in any case to have a similar chat. So time is added which can make a difference to the choice of treatments that are possible if the woman does want to end the pregnancy.
GPs also don’t always know what modern termination of pregnancy services offer- so a GP may not always be able to talk through options very knowledgeably. (like that the law has very recently changed to allow women in England the chance to take the second part of the early medical abortion treatment- which effectively starts the uterus to expel the pregnancy- at home.... making this a more realistic option for women who have to travel a long way to get to the specialist service premises, or have other caring commitments, or a limited budget to afford travel- since she’ll no longer be required to make two visits, close together, to the abortion services so that she can be given the first and then the second part of the treatment.
This update to the service combined with self referral probably means that more women at an early enough stage in pregnancy can have more treatment choice than they would otherwise have had.
That means that if they wish, they can avoid a more physically invasive surgical abortion treatment and anaesthetic risk. (Low risk relatively but this may be important to some women). Some women may just prefer to be at home to pass the pregnancy.