Mmm,
An interesting problem.
Just to say that Premiair and Eurocopter UK (ex McAlpine) are not one and the same. So check both.
Another possible is Specair. They have just taken over a police contract for EC135 flying and have taken on a 135 rated TRE.
Just so you appreciate the problem, you need an outfit that both has a TRTO for the EC135 AND a helicopter. Many TRTOs effectively do not have an EC135. They use the customers.
You may be able to make something come together if you can find a privately operated EC135 that is prepared to lease its aircaft for training. There are 1 or 2 possibles. If you go to an EC135 TRTO WITH a hired aircraft most will do the rating for you. (Bond are likely to just be too busy with ratings AND IRs)
Presumably you are going to fly for someone - if so, won't they let you "borrow" the aircraft for a day or two?
Another thought - try talking to Aeromega at Stapleford or Cambridge. A great bunch who will help if they can. Their MD is now/will be shortly the Chief Pilot for a police consortium. However, he is an EC135 TRE and between the company and the consortium they may be able to help. Again though you having the aircraft would help.