Not 100% sure, but I think that Starspeed may not operate the single Squirrel.
Possibles are Cabair, Elite and maybe London Heles
Many of the "leading" London outfits don't charter singles much these days.
Why do you specifically want an AS350? For charter it would need floats for Battersea.
If you mean you want to hire an AS350 for self-fly, that is another question.