What very often happens is that solicitors book counsel (a barrister) to deal with a matter - normally a few days in advance. However other cases the barristers are involved in run over (normally trials) and so they have to "return" the brief (a trial always takes precedence over a "new" brief, as do "part heard" cases). This means that very often Chambers is faced with reallocating briefs the night before to different counsel or they send it out to other chambers so the case is covered - this is why you couldn't have your "chosen" barrister. This said, barristers are quite used to picking up a brief the night before and being able to prepare it fully and properly (this is the way the system works and has always worked) and ensure the client is properly represented. I doubt very much that this has affected your defence. However I am, like you, concerned that your solicitor is claiming legal aid for meetings that have not taken place (I do not know how solicitors bill their legal aid) but barristers are on a graduated fee basis which is paid through the Court service so it won't matter to the barrister whether you spent 5 hours or 5 minutes, the fee will be the same for "in court" work.
I suggest you take further legal advice from a different firm of solicitors (for which you will have to pay). However, it will not affect the outcome.