I believe sponsorships are a bit thin on the ground in the UK. Britannia are in the process of selecting the few as we type.
I would wade around the caa srg website for the conversion info. It depends on if you got "wings" or any thing like that. If you haven't then it will be an expensive process.
Most are advising the modular route at the moment to minimise the risk to yourself. 400 hours isn't bad, if they are turbine hours then all the better. At the moment minimums are running about 1000-1500 hours with turbine time being very useful.
To cap,
Medical, shouldn't be a problem.
ATPL exams. 14 of them.
IR, 15 hours min if you have an ICAO one already.
CPL.