I went to Liverpool last year but fly into Manchester and took a train to Liverpool. Previously, KLM do fly direct to Liverpool with transit in Amsterdam.
It is better to fly into Manchester as the train ride is shorter compared to London. Plus London Heathrow airport tax is one of the highest in the region, that is what my friend told me. [Train from Liverpool lime st station to King Cross Station in London is slightly more than 2 hours ride per way]

It has been years since i took MAS, I wanted to pre-select my seats and apparently now MAS charged for seat selection too. I bought my flight ticket via an online portal and not directly with MAS