Centreline and Ravenair both have dispensations from the CAA to employ 400hr pilots but they stil require the multi engine IFR time and all the other tricky details. There are others such as Dragonfly and Woodgate that operate single crew aircraft as multi-crew under their AOC's, so there may be FO'ing opportunities for low houred pilots. Also much of the work conducted by charter operators is contract work i.e. regular freight runs carrying bank cheques between X and Y, not charters all over the place.
Air Taxi/Charter in light twins is a small industry in the UK. Unless you have plenty of unusual flying in your logbook, a lot of actual IFR and some decent contacts you would stand more chance of landing a sponsored position with BA than succeeding via this route.
It is a small industry sector but if you have the hours and are capable of passing the base check with just an hour or so practice, the work is there. Not that many aircraft or operators, but very few pilots with the hours chasing the jobs and people moving on fast to airlines........But plenty of cowboy operators out there (see Unique Air thread in biz jets) so be very careful.