More importantly, who will let you fly their twin after gaining the MEP?
Numerous schools will train you for the MEP, but won't let you take their precious twin on your own, or are too booked up with commercials and IR's.
I gained my MEP in Florida and had these problems when returning to the UK. The best solution I found was
www.theflyinghut.co.uk, who have a Piper Aztec.
After sufficient training, not only do they let you take the Aztec solo, but will let you take it abroad, overnight, for the day etc etc.
Compared to the duchess, seneca etc the aztec is alot faster (160kt cruise), requires alot more thinking ahead, is alot heavier, bigger, carries more passengers with full fuel (ie 6). I found it is really progressing my flying.