Like most brokers in the UK, we use the RSP network to make our trades so..
Non-order book trading
Private investors trade shares through stockbrokers. Brokers typically request prices from a number of market makers called RSPs who offer prices in “retail” (relatively small) sized volumes.
The price supplied to you by a broker is the best price they have been quoted by the RSPs they deal with. These, in turn, are based on the price available on the London Stock Exchange’s order books. Traditionally, you can only trade via your broker at the price supplied by an RSP and unexecuted limit orders are warehoused by the broker and not displayed to the entire market.
quote from the London Stock Echange’s website
When it comes to best execution, we’re required by the regulator to provide that for our users which means -
Best execution is a standard which takes into account a number of factors including price, certainty of settlement and speed. But fundamentally, it means we do our utmost to get you the best available price on the market at the time.
quote from this blog post
I hope that helps!