Hi anxiao,
Be aware some of the companies mentioned above are brokers rather than operators, and as such will add commission.
Going back a few years, I used to arrange regular charters with fixed wing (and rotary) operators, and would definitely recommend Synergy Aviation - they were always our 'go to' company for smaller aircraft.
Others such as 2Excel and SaxonAir are definitely worth a look too, and some of the other bigger operators such as Gama, Bookajet, Centreline do operate smaller aircraft too (I have arranged charters with all of these in the past, except 2Excel).
No personal experience, but there are some smaller operators too, such as Alto Aerospace, Eagle European, Zephr Aviation, all with UK-issued AOCs, and may be more cost effective options.
One last suggestion, I understand that EscapeTime (part of British European Aviation, who also own HeliAir) have recently been granted an AOC for SET-IMC charters in a PA46 M600. Not sure of the range to be honest, but as it's a single could be the most cost effective option if you're comfortable with that.
Hope the above helps!
h14