JAR's FAR's, whatever.
If you intend to log these twin hours and come back to the UK to seek employment with a UK operator just ask yourself this question;
I have to pass the interview to be offered the sim ride where I can impress them to hell with my flying ability. One question that is absolutely, positvely, definately going to be asked of you at interview is where did you get your twin hours. If you have 1000 from flying air taxi you have no problem. If you have 100 out of a total time of less than 500 and they were all bought in the US whilst you were acting as 'second pilot' and not actually flying the thing then you may have a hard time convincing them of your merits. Rightly or wrongly the interviewer is UK based and most probably biased against this practice. Do you want to get the chance to sim test or not?
Better spending you money on an instructor rating and then teach, go para-dropping etc to get some 'commercial' flying under the belt and keep pestering the school if they have twins, they may let you position them for maintenance etc etc and then you'll be getting the odd twin hour here and there. Make contact with the air taxi outfits and keep in touch with them personally, right place, right time and all that.
Good luck, think hard before parting with the greenbacks!
PP