G'day Africabynight.
I was in a similar position to you and my time was on Bandierantes and Twin Otters. Both are JAR 23 not JAR 25. For your license conversion, you won't be able to use the B1900 as it is considered to be a single pilot aircraft. See
JAA.
To be awarded an exemption from the MCC, you'll need a letter from your employer validating your hours and operations. You should also get a copy of the regulations so you can quote it to the UK CAA as well.
Unfortunately, while your hours are good, they aren't fabulous with respect to jet & larger turboprop operators and so forth so to expect a hand financially is a pipe dream. Although, never say never, contact airlines such as Air Southwest and see what they say. However, I wouldn't get my hopes up.
However, what you can do once you have 500 hours on the Beech 1900 is by-pass the IR and COM flying and go straight onto a JAR 25 type rating with the LST observed by a CAA examiner. This way you're awarded a JAA ATPL straight away. Review
LASORS as well.
Yes, it is frustrating and annoying.