Ok, to keep the CAA happy they will want a letter from your company validating those hours. It has to be on company letter head paper and have your dates of employment plus hours attained.
Also, the letter must state that the aircraft was flown under multi pilot ops because of local regulations. I have forgotten the CAO it comes under. (If the B1900 is a single pilot aircraft, I don't think it is)
One thing you might want to check if the B1900 is acceptable to the CAA for the ATPL LST. If it is, then just contact Flight Safety at Farnborough, there's a B1900 sim there.
I am not sure if there are any Dash 8 sims in the UK. I could be wrong but I think you might need to look at Paris or Sweden?? Remember, you need to get the LST observed by the CAA (assuming you're using the UK as a starting point) and this will be very expensive to organise outside of the UK. (Business class airfares, hotels etc)
As for a cheap JAR 25, you used to be able to do it on a Shorts 360 but I wouldn't advise that now. I would contact FSI at Farnborough and see if they have any spare seats on a Hawker 800 or Falcon. Price might suit.
Failing that, A320 or B737 are your best bet to get the licence.
Now, bear in mind that you'll need to do base training (ie 4-6 circuits) to finish the job. £££.
Therefore, as much as I hate to say it, the boring old CPL/IR may be the most cost effective and cheapest way to go.