Should Maria be signing in Makaton or BSL?

I've met several children with RTS who have the same problem as Maria: they can understand spoken language but they have difficulty speaking it. This is usually expressed as good receptive but poor expressive language skills.

In situations like these it's common for the experts (Educational Psychologist, Speech and Language Therapist) to recommend other modes of language expression (the jargon is augmentative and alternative communication, abbreviated to AAC) which include sign systems (like Makaton and Singalong) and symbolic systems (like PECS and Proloquo2go). These systems are not as nuanced as a full language but they do provide a basic means of communication. Also sign systems are much quicker to use than symbolic systems when communicating with someone.

It's quite common nowadays for parents to use baby sign language from the age of one onwards. This helps young childen express basic needs such as hunger, thirst and toilet well before they can speak. Maria was about three years old when she started to baby sign and was very keen to learn, picking up her first ten signs in less than thirty minutes. We used the inspirational Sing and Sign DVDs by Sasha Felix which teach a mixture of Makaton and Signalong signs. Maria and Isabella loved to watch these DVDs and Maria knew them backwards (she could turn her back to the TV and sign any bit of the video). It was no accident that I bought these DVDs; at the age of two I could sense Maria's need to express herself and by three years she was clearly getting frustrated. At age three she'd been babbling for over a year and was using gestures to communicate, so signing seemed a natural extension of this behaviour.  Little did I realise just how important Makaton would become as Maria's first expressive language.

I knew from Cathy Stevens (2011) that at least 15% of adults with RTS develop no speech. At that time (2 to 4 years old) Maria was showing little aptitude for speech and so I decided to have a backup plan just in case she became one of the 15% that never learnt to speak. Baby signing followed by Makaton seemed like a good choice for now but what if Maria reached the age of six and still couldn't speak? Was she supposed to use Makaton for the rest of her life? Didn't Maria deserve to be taught a proper language to express herself? If so then which language should it be?

The main sign language of the UK is British Sign Language (BSL) but when I suggested using this to the 'experts' (the Educational Psychologist, the LEA Head of SEN and the London deaf school) they all said 'no' for different reasons.The Educational Psychologist thought it was the wrong choice and insisted it should be Makaton (but was sure Maria would speak anyway). The LEA Head of SEN told me that only deaf children are permitted access to BSL schools; and the London deaf school told me they only take deaf children. 

All these opinions got me researching some of the sign systems and sign languages used by people in the UK to see which was the best choice for Maria in her later years, should she never develop speech.