naomi / webbo, there is an ongoing trial to use online photo's. I do not know how it works and it is restricted to the few chosen trial areas of the UK.
The best place to get Passport photo's is Tesco photoshop. They take a photo and scan it direct to the Passport office website. They then get an instant Yes/No answer as to if it is acceptable or not. They will repeat the photo free of charge until they get a 'Yes' from the passport office.
You still have to get the photo & ID signed and sent off by post though.